Source code for pennylane.optimize.adam

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#     http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Adam optimizer"""
from numpy import sqrt

from .gradient_descent import GradientDescentOptimizer


[docs]class AdamOptimizer(GradientDescentOptimizer):
    r"""Gradient-descent optimizer with adaptive learning rate, first and second moment.

    Adaptive Moment Estimation uses a step-dependent learning rate,
    a first moment :math:`a` and a second moment :math:`b`, reminiscent of
    the momentum and velocity of a particle:

    .. math::
        x^{(t+1)} = x^{(t)} - \eta^{(t+1)} \frac{a^{(t+1)}}{\sqrt{b^{(t+1)}} + \epsilon },

    where the update rules for the two moments are given by

    .. math::
        a^{(t+1)} &= \beta_1 a^{(t)} + (1-\beta_1) \nabla f(x^{(t)}),\\
        b^{(t+1)} &= \beta_2 b^{(t)} + (1-\beta_2) (\nabla f(x^{(t)}))^{\odot 2},\\
        \eta^{(t+1)} &= \eta \frac{\sqrt{(1-\beta_2^{t+1})}}{(1-\beta_1^{t+1})}.

    Above, :math:`( \nabla f(x^{(t-1)}))^{\odot 2}` denotes the element-wise square operation,
    which means that each element in the gradient is multiplied by itself. The hyperparameters
    :math:`\beta_1` and :math:`\beta_2` can also be step-dependent. Initially, the first and
    second moment are zero.

    The shift :math:`\epsilon` avoids division by zero.

    For more details, see `arXiv:1412.6980 <https://arxiv.org/abs/1412.6980>`_.

    Args:
        stepsize (float): the user-defined hyperparameter :math:`\eta`
        beta1 (float): hyperparameter governing the update of the first and second moment
        beta2 (float): hyperparameter governing the update of the first and second moment
        eps (float): offset :math:`\epsilon` added for numerical stability

    .. note::

        When using ``torch``, ``tensorflow`` or ``jax`` interfaces, refer to :doc:`Gradients and training </introduction/interfaces>` for suitable optimizers.

    """

    def __init__(self, stepsize=0.01, beta1=0.9, beta2=0.99, eps=1e-8):
        super().__init__(stepsize)
        self.beta1 = beta1
        self.beta2 = beta2
        self.eps = eps
        self.accumulation = None

[docs]    def apply_grad(self, grad, args):
        r"""Update the variables args to take a single optimization step. Flattens and unflattens
        the inputs to maintain nested iterables as the parameters of the optimization.

        Args:
            grad (tuple[ndarray]): the gradient of the objective
                function at point :math:`x^{(t)}`: :math:`\nabla f(x^{(t)})`
            args (tuple): the current value of the variables :math:`x^{(t)}`

        Returns:
            list: the new values :math:`x^{(t+1)}`
        """
        args_new = list(args)

        if self.accumulation is None:
            self.accumulation = {"fm": [0] * len(args), "sm": [0] * len(args), "t": 0}

        self.accumulation["t"] += 1

        # Update step size (instead of correcting for bias)
        new_stepsize = (
            self.stepsize
            * sqrt(1 - self.beta2 ** self.accumulation["t"])
            / (1 - self.beta1 ** self.accumulation["t"])
        )

        trained_index = 0
        for index, arg in enumerate(args):
            if getattr(arg, "requires_grad", False):
                self._update_accumulation(index, grad[trained_index])
                args_new[index] = arg - new_stepsize * self.accumulation["fm"][index] / (
                    sqrt(self.accumulation["sm"][index]) + self.eps
                )

                trained_index += 1

        return args_new

    def _update_accumulation(self, index, grad):
        r"""Update the moments.

        Args:
            index (int): the index of the argument to update
            grad (ndarray): the gradient for that trainable param
        """
        # update first moment
        self.accumulation["fm"][index] = (
            self.beta1 * self.accumulation["fm"][index] + (1 - self.beta1) * grad
        )

        # update second moment
        self.accumulation["sm"][index] = (
            self.beta2 * self.accumulation["sm"][index] + (1 - self.beta2) * grad**2
        )

[docs]    def reset(self):
        """Reset optimizer by erasing memory of past steps."""
        self.accumulation = None

    @property
    def fm(self):
        """Returns estimated first moments of gradient"""
        return None if self.accumulation is None else self.accumulation["fm"]

    @property
    def sm(self):
        """Returns estimated second moments of gradient"""
        return None if self.accumulation is None else self.accumulation["sm"]

    @property
    def t(self):
        """Returns accumulated timesteps"""
        return None if self.accumulation is None else self.accumulation["t"]
Source code for pennylane.optimize.adam

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