Source code for pennylane.qnn.keras

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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#
#     http://www.apache.org/licenses/LICENSE-2.0
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"""This module contains the classes and functions for integrating QNodes with the Keras Layer
API."""
import inspect
from collections.abc import Iterable
from typing import Optional, Union, Sequence, Text

from pennylane.transforms.batch_input import batch_input

try:
    import tensorflow as tf
    from tensorflow.keras.layers import Layer

    CORRECT_TF_VERSION = int(tf.__version__.split(".", maxsplit=1)[0]) > 1
except ImportError:
    # The following allows this module to be imported even if TensorFlow is not installed. Users
    # will instead see an ImportError when instantiating the KerasLayer.
    from abc import ABC

    Layer = ABC
    CORRECT_TF_VERSION = False


[docs]class KerasLayer(Layer): """KerasLayer(qnode, weight_shapes: dict, output_dim, weight_specs: Optional[dict] = None, **kwargs) Converts a :func:`~.QNode` to a Keras `Layer <https://www.tensorflow.org/api_docs/python/tf/keras/layers/Layer>`__. The result can be used within the Keras `Sequential <https://www.tensorflow.org/api_docs/python/tf/keras/Sequential>`__ or `Model <https://www.tensorflow.org/api_docs/python/tf/keras/Model>`__ classes for creating quantum and hybrid models. Args: qnode (qml.QNode): the PennyLane QNode to be converted into a Keras Layer_ weight_shapes (dict[str, tuple]): a dictionary mapping from all weights used in the QNode to their corresponding shapes output_dim (int): the output dimension of the QNode weight_specs (dict[str, dict]): An optional dictionary for users to provide additional specifications for weights used in the QNode, such as the method of parameter initialization. This specification is provided as a dictionary with keys given by the arguments of the `add_weight() <https://www.tensorflow.org/api_docs/python/tf/keras/layers/Layer#add_weight>`__ method and values being the corresponding specification. batch_idx (Union[Sequence[int], int]): Argument location of the non-trainable inputs for the circuit. This allows batch execution by creating executable circuits for each input example with the same trainable weights. Default ``None``. See :func:`~.pennylane.transforms.batch_input` for more details. **kwargs: additional keyword arguments passed to the Layer_ base class **Example** First let's define the QNode that we want to convert into a Keras Layer_: .. code-block:: python n_qubits = 2 dev = qml.device("default.qubit", wires=n_qubits) @qml.qnode(dev) def qnode(inputs, weights_0, weight_1): qml.RX(inputs[0], wires=0) qml.RX(inputs[1], wires=1) qml.Rot(*weights_0, wires=0) qml.RY(weight_1, wires=1) qml.CNOT(wires=[0, 1]) return qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliZ(1)) The signature of the QNode **must** contain an ``inputs`` named argument for input data, with all other arguments to be treated as internal weights. We can then convert to a Keras Layer_ with: >>> weight_shapes = {"weights_0": 3, "weight_1": 1} >>> qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2) The internal weights of the QNode are automatically initialized within the :class:`~.KerasLayer` and must have their shapes specified in a ``weight_shapes`` dictionary. It is then easy to combine with other neural network layers from the `tensorflow.keras.layers <https://www.tensorflow.org/api_docs/python/tf/keras/layers>`__ module and create a hybrid: >>> clayer = tf.keras.layers.Dense(2) >>> model = tf.keras.models.Sequential([qlayer, clayer]) .. details:: :title: Usage Details **QNode signature** The QNode must have a signature that satisfies the following conditions: - Contain an ``inputs`` named argument for input data. - All other arguments must accept an array or tensor and are treated as internal weights of the QNode. - All other arguments must have no default value. - The ``inputs`` argument is permitted to have a default value provided the gradient with respect to ``inputs`` is not required. - There cannot be a variable number of positional or keyword arguments, e.g., no ``*args`` or ``**kwargs`` present in the signature. **Initializing weights** The optional ``weight_specs`` argument of :class:`~.KerasLayer` allows for a more fine-grained specification of the QNode weights, such as the method of initialization and any regularization or constraints. For example, the initialization method of the ``weights`` argument in the example above could be specified by: .. code-block:: weight_specs = {"weights": {"initializer": "random_uniform"}} The values of ``weight_specs`` are dictionaries with keys given by arguments of the Keras `add_weight() <https://www.tensorflow.org/api_docs/python/tf/keras/layers/Layer#add_weight>`__ method. For the ``"initializer"`` argument, one can specify a string such as ``"random_uniform"`` or an instance of an `Initializer <https://www.tensorflow.org/api_docs/python/tf/keras/initializers>`__ class, such as `tf.keras.initializers.RandomUniform <https://www.tensorflow.org/api_docs/python/tf/random_uniform_initializer>`__. If ``weight_specs`` is not specified, weights will be added using the Keras default initialization and without any regularization or constraints. **Additional example** The code block below shows how a circuit composed of templates from the :doc:`/introduction/templates` module can be combined with classical `Dense <https://www.tensorflow.org/api_docs/python/tf/keras/layers/Dense>`__ layers to learn the two-dimensional `moons <https://scikit-learn.org/stable/modules/generated/sklearn .datasets.make_moons.html>`__ dataset. .. code-block:: python import pennylane as qml import tensorflow as tf import sklearn.datasets n_qubits = 2 dev = qml.device("default.qubit", wires=n_qubits) @qml.qnode(dev) def qnode(inputs, weights): qml.templates.AngleEmbedding(inputs, wires=range(n_qubits)) qml.templates.StronglyEntanglingLayers(weights, wires=range(n_qubits)) return qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliZ(1)) weight_shapes = {"weights": (3, n_qubits, 3)} qlayer = qml.qnn.KerasLayer(qnode, weight_shapes, output_dim=2) clayer1 = tf.keras.layers.Dense(2) clayer2 = tf.keras.layers.Dense(2, activation="softmax") model = tf.keras.models.Sequential([clayer1, qlayer, clayer2]) data = sklearn.datasets.make_moons() X = tf.constant(data[0]) Y = tf.one_hot(data[1], depth=2) opt = tf.keras.optimizers.SGD(learning_rate=0.5) model.compile(opt, loss='mae') The model can be trained using: >>> model.fit(X, Y, epochs=8, batch_size=5) Train on 100 samples Epoch 1/8 100/100 [==============================] - 9s 90ms/sample - loss: 0.3524 Epoch 2/8 100/100 [==============================] - 9s 87ms/sample - loss: 0.2441 Epoch 3/8 100/100 [==============================] - 9s 87ms/sample - loss: 0.1908 Epoch 4/8 100/100 [==============================] - 9s 87ms/sample - loss: 0.1832 Epoch 5/8 100/100 [==============================] - 9s 88ms/sample - loss: 0.1596 Epoch 6/8 100/100 [==============================] - 9s 87ms/sample - loss: 0.1637 Epoch 7/8 100/100 [==============================] - 9s 86ms/sample - loss: 0.1613 Epoch 8/8 100/100 [==============================] - 9s 87ms/sample - loss: 0.1474 **Returning a state** If your QNode returns the state of the quantum circuit using :func:`~.state` or :func:`~.density_matrix`, you must immediately follow your quantum Keras Layer with a layer that casts to reals. For example, you could use `tf.keras.layers.Lambda <https://www.tensorflow.org/api_docs/python/tf/keras/layers/Lambda>`__ with the function ``lambda x: tf.abs(x)``. This casting is required because TensorFlow's Keras layers require a real input and are differentiated with respect to real parameters. .. _Layer: https://www.tensorflow.org/api_docs/python/tf/keras/layers/Layer """ def __init__( self, qnode, weight_shapes: dict, output_dim, weight_specs: Optional[dict] = None, batch_idx: Union[Sequence[int], int] = None, **kwargs, ): # pylint: disable=too-many-arguments if not CORRECT_TF_VERSION: raise ImportError( "KerasLayer requires TensorFlow version 2 or above. The latest " "version of TensorFlow can be installed using:\n" "pip install tensorflow --upgrade\nAlternatively, visit " "https://www.tensorflow.org/install for detailed instructions." ) self.weight_shapes = { weight: (tuple(size) if isinstance(size, Iterable) else (size,) if size > 1 else ()) for weight, size in weight_shapes.items() } self._signature_validation(qnode, weight_shapes) self.argnum = batch_idx if batch_idx is None: self.qnode = qnode else: self.qnode = batch_input(qnode, argnum=batch_idx) self.qnode.interface = "tf" # Allows output_dim to be specified as an int or as a tuple, e.g, 5, (5,), (5, 2), [5, 2] # Note: Single digit values will be considered an int and multiple as a tuple, e.g [5,] or (5,) # are passed as integer 5 and [5, 2] will be passes as tuple (5, 2) if isinstance(output_dim, Iterable) and len(output_dim) > 1: self.output_dim = tuple(output_dim) else: self.output_dim = output_dim[0] if isinstance(output_dim, Iterable) else output_dim self.weight_specs = weight_specs if weight_specs is not None else {} self.qnode_weights = {} super().__init__(dynamic=True, **kwargs) def _signature_validation(self, qnode, weight_shapes): sig = inspect.signature(qnode.func).parameters if self.input_arg not in sig: raise TypeError( f"QNode must include an argument with name {self.input_arg} for inputting data" ) if self.input_arg in set(weight_shapes.keys()): raise ValueError( f"{self.input_arg} argument should not have its dimension specified in " f"weight_shapes" ) param_kinds = [p.kind for p in sig.values()] if inspect.Parameter.VAR_POSITIONAL in param_kinds: raise TypeError("Cannot have a variable number of positional arguments") if inspect.Parameter.VAR_KEYWORD not in param_kinds: if set(weight_shapes.keys()) | {self.input_arg} != set(sig.keys()): raise ValueError("Must specify a shape for every non-input parameter in the QNode")
[docs] def build(self, input_shape): """Initializes the QNode weights. Args: input_shape (tuple or tf.TensorShape): shape of input data """ for weight, size in self.weight_shapes.items(): spec = self.weight_specs.get(weight, {}) self.qnode_weights[weight] = self.add_weight(name=weight, shape=size, **spec) super().build(input_shape)
[docs] def call(self, inputs): """Evaluates the QNode on input data using the initialized weights. Args: inputs (tensor): data to be processed Returns: tensor: output data """ if len(tf.shape(inputs)) > 1 and self.argnum is None: # If the input size is not 1-dimensional, unstack the input along its first dimension, # recursively call the forward pass on each of the yielded tensors, and then stack the # outputs back into the correct shape reconstructor = [] for x in tf.unstack(inputs): reconstructor.append(self.call(x)) return tf.stack(reconstructor) return self._evaluate_qnode(inputs)
def _evaluate_qnode(self, x): """Evaluates a QNode for a single input datapoint. Args: x (tensor): the datapoint Returns: tensor: output datapoint """ kwargs = { **{self.input_arg: x}, **{k: 1.0 * w for k, w in self.qnode_weights.items()}, } return self.qnode(**kwargs)
[docs] def compute_output_shape(self, input_shape): """Computes the output shape after passing data of shape ``input_shape`` through the QNode. Args: input_shape (tuple or tf.TensorShape): shape of input data Returns: tf.TensorShape: shape of output data """ return tf.TensorShape([input_shape[0]]).concatenate(self.output_dim)
def __str__(self): detail = "<Quantum Keras Layer: func={}>" return detail.format(self.qnode.func.__name__) __repr__ = __str__ _input_arg = "inputs" @property def input_arg(self): """Name of the argument to be used as the input to the Keras `Layer <https://www.tensorflow.org/api_docs/python/tf/keras/layers/Layer>`__. Set to ``"inputs"``.""" return self._input_arg
[docs] @staticmethod def set_input_argument(input_name: Text = "inputs") -> None: """ Set the name of the input argument. Args: input_name (str): Name of the input argument """ KerasLayer._input_arg = input_name
[docs] def get_config(self) -> dict: """ Get serialized layer configuration Returns: dict: layer configuration """ config = super().get_config() config.update( { "output_dim": self.output_dim, "weight_specs": self.weight_specs, "weight_shapes": self.weight_shapes, "argnum": self.argnum, } ) return config