qml.GradientDescentOptimizer

class GradientDescentOptimizer(stepsize=0.01)[source]

Bases: object

Basic gradient-descent optimizer.

Base class for other gradient-descent-based optimizers.

A step of the gradient descent optimizer computes the new values via the rule

\[x^{(t+1)} = x^{(t)} - \eta \nabla f(x^{(t)}).\]

where \(\eta\) is a user-defined hyperparameter corresponding to step size.

Parameters

stepsize (float) – the user-defined hyperparameter \(\eta\)

apply_grad(grad, args)

Update the variables to take a single optimization step.

compute_grad(objective_fn, args, kwargs[, …])

Compute gradient of the objective function at the given point and return it along with the objective function forward pass (if available).

step(objective_fn, *args[, grad_fn])

Update trainable arguments with one step of the optimizer.

step_and_cost(objective_fn, *args[, grad_fn])

Update trainable arguments with one step of the optimizer and return the corresponding objective function value prior to the step.

update_stepsize(stepsize)

Update the initialized stepsize value \(\eta\).

apply_grad(grad, args)[source]

Update the variables to take a single optimization step. Flattens and unflattens the inputs to maintain nested iterables as the parameters of the optimization.

Parameters
  • grad (tuple [array]) – the gradient of the objective function at point \(x^{(t)}\): \(\nabla f(x^{(t)})\)

  • args (tuple) – the current value of the variables \(x^{(t)}\)

Returns

the new values \(x^{(t+1)}\)

Return type

list [array]

static compute_grad(objective_fn, args, kwargs, grad_fn=None)[source]

Compute gradient of the objective function at the given point and return it along with the objective function forward pass (if available).

Parameters
  • objective_fn (function) – the objective function for optimization

  • args (tuple) – tuple of NumPy arrays containing the current parameters for the objection function

  • kwargs (dict) – keyword arguments for the objective function

  • grad_fn (function) – optional gradient function of the objective function with respect to the variables args. If None, the gradient function is computed automatically. Must return the same shape of tuple [array] as the autograd derivative.

Returns

NumPy array containing the gradient \(\nabla f(x^{(t)})\) and the objective function output. If grad_fn is provided, the objective function will not be evaluted and instead None will be returned.

Return type

tuple (array)

step(objective_fn, *args, grad_fn=None, **kwargs)[source]

Update trainable arguments with one step of the optimizer.

Parameters
  • objective_fn (function) – the objective function for optimization

  • *args – Variable length argument list for objective function

  • grad_fn (function) – optional gradient function of the objective function with respect to the variables x. If None, the gradient function is computed automatically. Must return a tuple[array] with the same number of elements as *args. Each array of the tuple should have the same shape as the corresponding argument.

  • **kwargs – variable length of keyword arguments for the objective function

Returns

the new variable values \(x^{(t+1)}\). If single arg is provided, list [array] is replaced by array.

Return type

list [array]

step_and_cost(objective_fn, *args, grad_fn=None, **kwargs)[source]

Update trainable arguments with one step of the optimizer and return the corresponding objective function value prior to the step.

Parameters
  • objective_fn (function) – the objective function for optimization

  • *args – variable length argument list for objective function

  • grad_fn (function) – optional gradient function of the objective function with respect to the variables *args. If None, the gradient function is computed automatically. Must return a tuple[array] with the same number of elements as *args. Each array of the tuple should have the same shape as the corresponding argument.

  • **kwargs – variable length of keyword arguments for the objective function

Returns

the new variable values \(x^{(t+1)}\) and the objective function output prior to the step. If single arg is provided, list [array] is replaced by array.

Return type

tuple[list [array], float]

update_stepsize(stepsize)[source]

Update the initialized stepsize value \(\eta\).

This allows for techniques such as learning rate scheduling.

Parameters

stepsize (float) – the user-defined hyperparameter \(\eta\)

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