Op is a graph object that defines and performs computations in a graph.
It has to define the following methods.
This method is responsible for creating output
Variables of a suitable symbolic
Typeto serve as the outputs of this
Op’s application. The
Variables found in
*inputsmust be operated on using Aesara’s symbolic language to compute the symbolic output
Variables. This method should put these outputs into an
Applyinstance, and return the
This method creates an
Applynode representing the application of the
Opon the inputs provided. If the
Opcannot be applied to these inputs, it must raise an appropriate exception.
The inputs of the
Applyinstance returned by this call must be ordered correctly: a subsequent
self.make_node(*apply.inputs)must produce something equivalent to the first
perform(node, inputs, output_storage)¶
This method computes the function associated to this
Applynode created by the
inputsis a list of references to data to operate on using non-symbolic statements, (i.e., statements in Python, NumPy).
output_storageis a list of storage cells where the variables of the computation must be put.
node: This is a reference to an
Applynode which was previously obtained via the
Op.make_node()method. It is typically not used in simple
Ops, but it contains symbolic information that could be required for complex
inputs: This is a list of data from which the values stored in
output_storageare to be computed using non-symbolic language.
output_storage: This is a list of storage cells where the output is to be stored. A storage cell is a one-element list. It is forbidden to change the length of the list(s) contained in
output_storage. There is one storage cell for each output of the
The data put in
output_storagemust match the type of the symbolic output. This is a situation where the
nodeargument can come in handy.
output_storageelements to persist between evaluations, or it may reset
output_storagecells to hold a value of
None. It can also pre-allocate some memory for the
Opto use. This feature can allow
Op.perform()to reuse memory between calls, for example. If there is something preallocated in the
output_storage, it will be of the good dtype, but can have the wrong shape and have any stride pattern.
This method must be determined by the inputs. That is to say, if it is evaluated once on inputs A and returned B, then if ever inputs C, equal to A, are presented again, then outputs equal to B must be returned again.
You must be careful about aliasing outputs to inputs, and making modifications to any of the inputs. See Views and inplace operations before writing a
Op.perform()implementation that does either of these things.
otheris also an
Truehere is a promise to the rewrite system that the other
Opwill produce exactly the same graph effects (e.g. from its
Op.perform()) as this one, given identical inputs. This means it will produce the same output values, it will destroy the same inputs (same
Op.destroy_map), and will alias outputs to the same inputs (same
Op.view_map). For more details, see Views and inplace operations.
If you set
__props__, this will be automatically generated.
Opinstances compare equal, then they must return the same hash value.
Equally important, this hash value must not change during the lifetime of self.
Opinstances should be immutable in this sense.
If you set
Op.__props__, this will be automatically generated.
Optional methods or attributes¶
Must be a tuple. Lists the name of the attributes which influence the computation performed. This will also enable the automatic generation of appropriate
__str__methods. Should be set to
()if you have no attributes that are relevant to the computation to generate the methods.
New in version 0.7.
If this member variable is an integer, then the default implementation of
nodewas returned by
Op.make_node(). Otherwise, the entire list of outputs will be returned, unless it is of length 1, where the single element will be returned by itself.
make_thunk(node, storage_map, compute_map, no_recycling, impl=None)¶
This function must return a thunk, that is a zero-arguments function that encapsulates the computation to be performed by this
Opon the arguments of the node.
- node –
Applyinstance The node for which a thunk is requested.
- storage_map – dict of lists This maps variables to a one-element lists holding the variable’s current value. The one-element list acts as pointer to the value and allows sharing that “pointer” with other nodes and instances.
- compute_map – dict of lists This maps variables to one-element lists holding booleans. If the value is 0 then the variable has not been computed and the value should not be considered valid. If the value is 1 the variable has been computed and the value is valid. If the value is 2 the variable has been garbage-collected and is no longer valid, but shouldn’t be required anymore for this call.
- no_recycling – WRITEME WRITEME
- impl – None, ‘c’ or ‘py’ Which implementation to use.
The returned function must ensure that is sets the computed variables as computed in the
Defining this function removes the requirement for
perform()or C code, as you will define the thunk for the computation yourself.
- node –
By default this is a convenience function which calls
make_node()with the supplied arguments and returns the result indexed by
default_output. This can be overridden by subclasses to do anything else, but must return either an Aesara
Variableor a list of
If you feel the need to override
__call__to change the graph based on the arguments, you should instead create a function that will use your
Opand build the graphs that you want and call that instead of the
infer_shape(fgraph, node, shapes)¶
This function is needed for shape rewrites.
shapesis a list with one tuple for each input of the
Applynode (which corresponds to the inputs of the
Op). Each tuple contains as many elements as the number of dimensions of the corresponding input. The value of each element is the shape (number of items) along the corresponding dimension of that specific input.
While this might sound complicated, it is nothing more than the shape of each input as symbolic variables (one per dimension).
The function should return a list with one tuple for each output. Each tuple should contain the corresponding output’s computed shape.
Implementing this method will allow Aesara to compute the output’s shape without computing the output itself, potentially sparing you a costly recomputation.
It is only used to have more information printed by the memory profiler. It makes it print the mega flops and giga flops per second for each apply node. It takes as inputs two lists: one for the inputs and one for the outputs. They contain tuples that are the shapes of the corresponding inputs/outputs.
This allows you to specify a more informative string representation of your
Op. If an
Ophas parameters, it is highly recommended to have the
__str__method include the name of the
Op’s parameters’ values.
If you set
__props__, this will be automatically generated. You can still override it for custom output.
By default when rewrites are enabled, we remove during function compilation
Applynodes whose inputs are all constants. We replace the
Applynode with an Aesara constant variable. This way, the
Applynode is not executed at each function call. If you want to force the execution of an
Opduring the function call, make do_constant_folding return False.
As done in the Alloc
Op, you can return False only in some cases by analyzing the graph from the node parameter.
debug_perform(node, inputs, output_storage)¶
Undefined by default.
If you define this function then it will be used instead of C code or
Op.perform()to do the computation while debugging (currently DebugMode, but others may also use it in the future). It has the same signature and contract as
Ops that cause trouble with DebugMode with their normal behaviour to adopt a different one when run under that mode. If your
Opdoesn’t have any problems, don’t implement this.
If you want your
Op to work with
aesara.gradient.grad() you also
need to implement the functions described below.
These are the function required to work with
Opbeing defined is differentiable, its gradient may be specified symbolically in this method. Both
output_gradientsare lists of symbolic Aesara
Variables and those must be operated on using Aesara’s symbolic language. The
Op.grad()method must return a list containing one
Variablefor each input. Each returned
Variablerepresents the gradient with respect to that input computed based on the symbolic gradients with respect to each output.
If the output is not differentiable with respect to an input then this method should be defined to return a variable of type
NullTypefor that input. Likewise, if you have not implemented the gradient computation for some input, you may return a variable of type
NullTypefor that input.
aesara.gradientcontains convenience methods that can construct the variable for you:
If an element of
output_gradientis of type
aesara.gradient.DisconnectedType, it means that the cost is not a function of this output. If any of the
Op’s inputs participate in the computation of only disconnected outputs, then
DisconnectedTypevariables for those inputs.
Op.grad()method is not defined, then Aesara assumes it has been forgotten. Symbolic differentiation will fail on a graph that includes this
It must be understood that the
Op.grad()method is not meant to return the gradient of the
Op.grad()is a helper function that computes terms that appear in gradients.
Ophas a single vector-valued output
yand a single vector-valued input
x, then the
Op.grad()method will be passed
xand a second vector
Jto be the Jacobian of
ywith respect to
Op.grad()method should return
Op.grad()method, it will set
zto be the gradient of the cost
Cwith respect to
y. If this
Opis the only
Opthat acts on
dot(J.T,z)is the gradient of C with respect to
x. If there are other
Ops that act on
aesara.grad()will have to add up the terms of
x’s gradient contributed by the other
In practice, an
Op’s input and output are rarely implemented as single vectors. Even if an
Op’s output consists of a list containing a scalar, a sparse matrix, and a 4D tensor, you can think of these objects as being formed by rearranging a vector. Likewise for the input. In this view, the values computed by the
Op.grad()method still represent a Jacobian-vector product.
In practice, it is probably not a good idea to explicitly construct the Jacobian, which might be very large and very sparse. However, the returned value should be equal to the Jacobian-vector product.
So long as you implement this product correctly, you need not understand what
aesara.gradient.grad()is doing, but for the curious the mathematical justification is as follows:
In essence, the
Op.grad()method must simply implement through symbolic
Variables and operations the chain rule of differential calculus. The chain rule is the mathematical procedure that allows one to calculate the total derivative of the final scalar symbolic
Cwith respect to a primitive symbolic
Variablex found in the list
Op.grad()method does this using
output_gradientswhich provides the total derivative of
Cwith respect to a symbolic
Variablethat is returned by the
Op(this is provided in
output_gradients), as well as the knowledge of the total derivative of the latter with respect to the primitive
Variable(this has to be computed).
In mathematics, the total derivative of a scalar variable with respect to a vector of scalar variables , i.e. the gradient, is customarily represented as the row vector of the partial derivatives, whereas the total derivative of a vector of scalar variables with respect to another , is customarily represented by the matrix of the partial derivatives, i.e. the Jacobian matrix. In this convenient setting, the chain rule says that the gradient of the final scalar variable with respect to the primitive scalar variables in through those in is simply given by the matrix product: .
Here, the chain rule must be implemented in a similar but slightly more complex setting: Aesara provides in the list
output_gradientsone gradient for each of the
Variables returned by the
Op. Where is one such particular
Variable, the corresponding gradient found in
output_gradientsand representing is provided with a shape similar to and thus not necessarily as a row vector of scalars. Furthermore, for each
Op’s list of input variables
inputs, the returned gradient representing must have a shape similar to that of
If the output list of the
Opis , then the list
output_gradientsis . If
inputsconsists of the list , then
Op.gradshould return the list , where (and can stand for multiple dimensions).
In other words,
Op.grad()does not return , but instead the appropriate dot product specified by the chain rule: . Both the partial differentiation and the multiplication have to be performed by
Aesara currently imposes the following constraints on the values returned by the
- They must be
- When they are types that have dtypes, they must never have an integer dtype.
The output gradients passed to
Op.grad()will also obey these constraints.
Integers are a tricky subject. Integers are the main reason for having
NullTypeor zero gradient. When you have an integer as an argument to your
Op.grad()method, recall the definition of a derivative to help you decide what value to return:
Suppose your function f has an integer-valued output. For most functions you’re likely to implement in Aesara, this means your gradient should be zero, because for almost all . (The only other option is that the gradient could be undefined, if your function is discontinuous everywhere, like the rational indicator function)
Suppose your function has an integer-valued input. This is a little trickier, because you need to think about what you mean mathematically when you make a variable integer-valued in Aesara. Most of the time in machine learning we mean “ is a function of a real-valued , but we are only going to pass in integer-values of ”. In this case, exists, so the gradient through should be the same whether is an integer or a floating point variable. Sometimes what we mean is “ is a function of an integer-valued , and is only defined where is an integer.” Since doesn’t exist, the gradient is undefined. Finally, many times in Aesara, integer valued inputs don’t actually affect the elements of the output, only its shape.
If your function has both an integer-valued input and an integer-valued output, then both rules have to be combined:
- If is defined at , then the input gradient is defined. Since would be equal to almost everywhere, the gradient should be zero (first rule).
- If is only defined where is an integer, then the gradient is undefined, regardless of what the gradient with respect to the output is.
- is a dot product between and . and are integers.
Since the output is also an integer, is a step function.
Its gradient is zero almost everywhere, so
Op.grad()should return zeros in the shape of and .
- is a dot product between and . is floating point and is an integer. In this case the output is floating point. It doesn’t matter that is an integer. We consider to still be defined at . The gradient is exactly the same as if were floating point.
- is the argmax of along axis . The gradient with respect to is undefined, because is not defined for floating point . How could you take an argmax along a fractional axis? The gradient with respect to is 0, because almost everywhere.
- is a vector with elements, each of which taking on
the value The
Op.grad()method should return
DisconnectedTypefor , because the elements of don’t depend on . Only the shape of depends on . You probably also want to implement a connection_pattern method to encode this.
- converts float into an integer. converts an integer into a float. If the final cost , then the gradient with respect to will be 0.5, even if is an integer. However, the gradient with respect to will be 0, because the output of is integer-valued.
- They must be
Sometimes needed for proper operation of
Returns a list of list of booleans.
Op.connection_pattern[input_idx][output_idx]is true if the elements of
inputs[input_idx]have an effect on the elements of
nodeparameter is needed to determine the number of inputs. Some
Ops such as
Subtensortake a variable number of inputs.
If no connection_pattern is specified,
aesara.gradient.grad()will assume that all inputs have some elements connected to some elements of all outputs.
This method conveys two pieces of information that are otherwise not part of the Aesara graph:
- Which of the
Op’s inputs are truly ancestors of each of the
Op’s outputs. Suppose an
Ophas two inputs, and , and outputs and . is not really an ancestor of , but it appears to be so in the Aesara graph.
- Whether the actual elements of each input/output are relevant to a
For example, the shape
Opdoes not read its input’s elements, only its shape metadata. should thus raise a disconnected input exception (if these exceptions are enabled). As another example, the elements of the
Op’s outputs are not affected by the shape arguments to the
Failing to implement this function for an
Opthat needs it can result in two types of incorrect behavior:
aesara.gradient.grad()erroneously raising a
TypeErrorreporting that a gradient is undefined.
aesara.gradient.grad()failing to raise a
ValueErrorreporting that an input is disconnected.
Even if connection_pattern is not implemented correctly, if
aesara.gradient.grad()returns an expression, that expression will be numerically correct.
- Which of the
Optional, to work with
This function implements the application of the R-operator on the function represented by your
Op. Let assume that function is , with input , applying the R-operator means computing the Jacobian of and right-multiplying it by , the evaluation point, namely: .
inputsare the symbolic variables corresponding to the value of the input where you want to evaluate the Jacobian, and
eval_pointsare the symbolic variables corresponding to the value you want to right multiply the Jacobian with.
Same conventions as for the
Op.grad()method hold. If your
Opis not differentiable, you can return None. Note that in contrast to the method
Op.R_op()you need to return the same number of outputs as there are outputs of the
Op. You can think of it in the following terms. You have all your inputs concatenated into a single vector . You do the same with the evaluation points (which are as many as inputs and of the shame shape) and obtain another vector . For each output, you reshape it into a vector, compute the Jacobian of that vector with respect to and multiply it by . As a last step you reshape each of these vectors you obtained for each outputs (that have the same shape as the outputs) back to their corresponding shapes and return them as the output of the