[Docs] Feature maps and QML reorganization

docs/development/draw.md

@@ -58,7 +58,7 @@ print(html_string(block)) # markdown-exec: hide
 ```python exec="on" source="material-block" html="1"
 from qadence import feature_map, hea, chain
 
-block = chain(feature_map(4, fm_type="tower"), hea(4,2))
+block = chain(feature_map(4, reupload_scaling="Tower"), hea(4,2))
 from qadence.draw import html_string # markdown-exec: hide
 print(html_string(block)) # markdown-exec: hide
 ```

docs/qml/index.md

@@ -1,68 +1,76 @@
-Variational algorithms on noisy devices and quantum machine learning (QML) [^1] in particular are
-the target applications for Qadence. For this purpose, the
-library offers both flexible symbolic expressions for the
-quantum circuit parameters via `sympy` (see [here](../tutorials/parameters.md) for more
-details) and native automatic differentiation via integration with
-[PyTorch](https://pytorch.org/) deep learning framework.
+Variational algorithms on noisy devices and quantum machine learning (QML)[^1] in particular are one of the main
+target applications for Qadence. For this purpose, the library offers both flexible symbolic expressions for the
+quantum circuit parameters via `sympy` (see [here](../tutorials/parameters.md) for more details) and native automatic
+differentiation via integration with [PyTorch](https://pytorch.org/) deep learning framework.
+
+Furthermore, Qadence offers a wide range of utilities for helping building and researching quantum machine learning algorithms, including:
+
+* [a set of constructors](qml_constructors.md) for circuits commonly used in quantum machine learning such as feature maps and ansatze
+* [a set of tools](ml_tools) for training and optimizing quantum neural networks and loading classical data into a QML algorithm
+
+## Some simple examples
 
 Qadence symbolic parameter interface allows to create
 arbitrary feature maps to encode classical data into quantum circuits
 with an arbitrary non-linear function embedding for the input values:
 
-```python exec="on" source="material-block" html="1" result="json" session="qml"
+```python exec="on" source="material-block" result="json" session="qml"
 import qadence as qd
 from qadence.operations import *
 import torch
 from sympy import acos
 
 n_qubits = 4
 
+# Example feature map, also directly available with the `feature_map` function
 fp = qd.FeatureParameter("phi")
-feature_map = qd.kron(RX(i, 2 * acos(fp)) for i in range(n_qubits))
+fm = qd.kron(RX(i, acos(fp)) for i in range(n_qubits))
 
 # the key in the dictionary must correspond to
 # the name of the assigned to the feature parameter
 inputs = {"phi": torch.rand(3)}
-samples = qd.sample(feature_map, values=inputs)
-print(samples)
+samples = qd.sample(fm, values=inputs)
+print(f"samples = {samples[0]}")  # markdown-exec: hide
 ```
 
 The [`constructors.feature_map`][qadence.constructors.feature_map] module provides
 convenience functions to build commonly used feature maps where the input parameter
-is encoded in the single-qubit gates rotation angle.
+is encoded in the single-qubit gates rotation angle. This function will be further
+demonstrated in the [QML constructors tutorial](qml_constructors.md).
 
 Furthermore, Qadence is natively integrated with PyTorch automatic differentiation engine thus
 Qadence quantum models can be used seamlessly in a PyTorch workflow.
 
 Let's create a quantum neural network model using the feature map just defined, a
-digital-analog variational ansatz and a simple observable $X(0) \otimes X(1)$. We
-use the convenience `QNN` quantum model abstraction.
+digital-analog variational ansatz ([also explained here](qml_constructors.md)) and a
+simple observable $X(0) \otimes X(1)$. We use the convenience `QNN` quantum model abstraction.
 
 ```python exec="on" source="material-block" result="json" session="qml"
 ansatz = qd.hea(n_qubits, strategy="sDAQC")
-circuit = qd.QuantumCircuit(n_qubits, feature_map, ansatz)
+circuit = qd.QuantumCircuit(n_qubits, fm, ansatz)
 observable = qd.kron(X(0), X(1))
 
 model = qd.QNN(circuit, observable)
 
 # NOTE: the `QNN` is a torch.nn.Module
 assert isinstance(model, torch.nn.Module)
+print(isinstance(model, torch.nn.Module)) # markdown-exec: hide
 ```
 
 Differentiation works the same way as any other PyTorch module:
 
-```python exec="on" source="material-block" html="1" result="json" session="qml"
+```python exec="on" source="material-block" result="json" session="qml"
 values = {"phi": torch.rand(10, requires_grad=True)}
 
 # the forward pass of the quantum model returns the expectation
 # value of the input observable
 out = model(values)
-print(f"Quantum model output: {out}")
+print(f"Quantum model output: \n{out}\n")  # markdown-exec: hide
 
 # you can compute the gradient with respect to inputs using
 # PyTorch autograd differentiation engine
 dout = torch.autograd.grad(out, values["phi"], torch.ones_like(out), create_graph=True)[0]
-print(f"First-order derivative w.r.t. the feature parameter: {dout}")
+print(f"First-order derivative w.r.t. the feature parameter: \n{dout}")
 
 # you can also call directly a backward pass to compute derivatives with respect
 # to the variational parameters and use it for implementing variational
@@ -74,12 +82,12 @@ To run QML on real devices, Qadence offers generalized parameter shift rules (GP
 for arbitrary quantum operations which can be selected when constructing the
 `QNN` model:
 
-```python exec="on" source="material-block" html="1" result="json" session="qml"
+```python exec="on" source="material-block" result="json" session="qml"
 model = qd.QNN(circuit, observable, diff_mode="gpsr")
 out = model(values)
 
 dout = torch.autograd.grad(out, values["phi"], torch.ones_like(out), create_graph=True)[0]
-print(f"First-order derivative w.r.t. the feature parameter: {dout}")
+print(f"First-order derivative w.r.t. the feature parameter: \n{dout}")
 ```
 
 See [here](../advanced_tutorials/differentiability.md) for more details on how the parameter

docs/qml/ml_tools.md

@@ -0,0 +1,213 @@
+## Dataloaders
+
+When using Qadence, you can supply classical data to a quantum machine learning
+algorithm by using a standard PyTorch `DataLoader` instance. Qadence also provides
+the `DictDataLoader` convenience class which allows
+to build dictionaries of `DataLoader`s instances and easily iterate over them.
+
+```python exec="on" source="material-block"
+import torch
+from torch.utils.data import DataLoader, TensorDataset
+from qadence.ml_tools import DictDataLoader
+
+def dataloader() -> DataLoader:
+    batch_size = 5
+    x = torch.linspace(0, 1, batch_size).reshape(-1, 1)
+    y = torch.sin(x)
+
+    dataset = TensorDataset(x, y)
+    return DataLoader(dataset, batch_size=batch_size)
+
+
+def dictdataloader() -> DictDataLoader:
+    batch_size = 5
+
+    keys = ["y1", "y2"]
+    dls = {}
+    for k in keys:
+        x = torch.rand(batch_size, 1)
+        y = torch.sin(x)
+        dataset = TensorDataset(x, y)
+        dataloader = DataLoader(dataset, batch_size=batch_size)
+        dls[k] = dataloader
+
+    return DictDataLoader(dls)
+
+n_epochs = 2
+
+# iterate standard DataLoader
+dl = dataloader()
+for i in range(n_epochs):
+    data = next(iter(dl))
+
+# iterate DictDataLoader
+ddl = dictdataloader()
+for i in range(n_epochs):
+    data = next(iter(ddl))
+
+```
+
+## Optimization routines
+
+For training QML models, Qadence also offers a few out-of-the-box routines for optimizing differentiable
+models, _e.g._ `QNN`s and `QuantumModel`, containing either *trainable* and/or *non-trainable* parameters
+(see [the parameters tutorial](../tutorials/parameters.md) for detailed information about parameter types):
+
+* [`train_with_grad`][qadence.ml_tools.train_with_grad] for gradient-based optimization using PyTorch native optimizers
+* [`train_gradient_free`][qadence.ml_tools.train_gradient_free] for gradient-free optimization using
+the [Nevergrad](https://facebookresearch.github.io/nevergrad/) library.
+
+These routines performs training, logging/printing loss metrics and storing intermediate checkpoints of models. In the following, we
+use `train_with_grad` as example but the code can be used directly with the gradient-free routine.
+
+As every other training routine commonly used in Machine Learning, it requires
+`model`, `data` and an `optimizer` as input arguments.
+However, in addition, it requires a `loss_fn` and a `TrainConfig`.
+A `loss_fn` is required to be a function which expects both a model and data and returns a tuple of (loss, metrics: `<dict>`), where `metrics` is a dict of scalars which can be customized too.
+
+```python exec="on" source="material-block"
+import torch
+from itertools import count
+cnt = count()
+criterion = torch.nn.MSELoss()
+
+def loss_fn(model: torch.nn.Module, data: torch.Tensor) -> tuple[torch.Tensor, dict]:
+    next(cnt)
+    x, y = data[0], data[1]
+    out = model(x)
+    loss = criterion(out, y)
+    return loss, {}
+
+```
+
+The [`TrainConfig`][qadence.ml_tools.config.TrainConfig] tells `train_with_grad` what batch_size should be used,
+how many epochs to train, in which intervals to print/log metrics and how often to store intermediate checkpoints.
+
+```python exec="on" source="material-block"
+from qadence.ml_tools import TrainConfig
+
+batch_size = 5
+n_epochs = 100
+
+config = TrainConfig(
+    folder="some_path/",
+    max_iter=n_epochs,
+    checkpoint_every=100,
+    write_every=100,
+    batch_size=batch_size,
+)
+```
+
+Let's see it in action with a simple example.
+
+### Fitting a funtion with a QNN using `ml_tools`
+
+Let's look at a complete example of how to use `train_with_grad` now.
+
+```python exec="on" source="material-block" html="1"
+from pathlib import Path
+import torch
+from itertools import count
+from qadence.constructors import hamiltonian_factory, hea, feature_map
+from qadence import chain, Parameter, QuantumCircuit, Z
+from qadence.models import QNN
+from qadence.ml_tools import train_with_grad, TrainConfig
+import matplotlib.pyplot as plt
+
+n_qubits = 2
+fm = feature_map(n_qubits)
+ansatz = hea(n_qubits=n_qubits, depth=3)
+observable = hamiltonian_factory(n_qubits, detuning=Z)
+circuit = QuantumCircuit(n_qubits, fm, ansatz)
+
+model = QNN(circuit, observable, backend="pyqtorch", diff_mode="ad")
+batch_size = 1
+input_values = {"phi": torch.rand(batch_size, requires_grad=True)}
+pred = model(input_values)
+
+cnt = count()
+criterion = torch.nn.MSELoss()
+optimizer = torch.optim.Adam(model.parameters(), lr=0.1)
+
+def loss_fn(model: torch.nn.Module, data: torch.Tensor) -> tuple[torch.Tensor, dict]:
+    next(cnt)
+    x, y = data[0], data[1]
+    out = model(x)
+    loss = criterion(out, y)
+    return loss, {}
+
+tmp_path = Path("/tmp")
+
+n_epochs = 50
+
+config = TrainConfig(
+    folder=tmp_path,
+    max_iter=n_epochs,
+    checkpoint_every=100,
+    write_every=100,
+    batch_size=batch_size,
+)
+
+batch_size = 25
+
+x = torch.linspace(0, 1, batch_size).reshape(-1, 1)
+y = torch.sin(x)
+
+train_with_grad(model, (x, y), optimizer, config, loss_fn=loss_fn)
+
+plt.clf() # markdown-exec: hide
+plt.plot(x.numpy(), y.numpy())
+plt.plot(x.numpy(), model(x).detach().numpy())
+from docs import docsutils # markdown-exec: hide
+print(docsutils.fig_to_html(plt.gcf())) # markdown-exec: hide
+```
+
+For users who want to use the low-level API of `qadence`, here is the example from above
+written without `train_with_grad`.
+
+### Fitting a function - Low-level API
+
+```python exec="on" source="material-block"
+from pathlib import Path
+import torch
+from itertools import count
+from qadence.constructors import hamiltonian_factory, hea, feature_map
+from qadence import chain, Parameter, QuantumCircuit, Z
+from qadence.models import QNN
+from qadence.ml_tools import train_with_grad, TrainConfig
+
+n_qubits = 2
+fm = feature_map(n_qubits)
+ansatz = hea(n_qubits=n_qubits, depth=3)
+observable = hamiltonian_factory(n_qubits, detuning=Z)
+circuit = QuantumCircuit(n_qubits, fm, ansatz)
+
+model = QNN(circuit, observable, backend="pyqtorch", diff_mode="ad")
+batch_size = 1
+input_values = {"phi": torch.rand(batch_size, requires_grad=True)}
+pred = model(input_values)
+
+criterion = torch.nn.MSELoss()
+optimizer = torch.optim.Adam(model.parameters(), lr=0.1)
+n_epochs=50
+cnt = count()
+
+tmp_path = Path("/tmp")
+
+config = TrainConfig(
+    folder=tmp_path,
+    max_iter=n_epochs,
+    checkpoint_every=100,
+    write_every=100,
+    batch_size=batch_size,
+)
+
+x = torch.linspace(0, 1, batch_size).reshape(-1, 1)
+y = torch.sin(x)
+
+for i in range(n_epochs):
+    out = model(x)
+    loss = criterion(out, y)
+    loss.backward()
+    optimizer.step()
+```

docs/qml/qaoa.md

@@ -140,7 +140,7 @@ for i in range(n_epochs):
 ```
 
 Qadence offers some convenience functions to implement this training loop with advanced
-logging and metrics track features. You can refer to [this](../qml/qml_tools.md) for more details.
+logging and metrics track features. You can refer to [this tutorial](../qml/ml_tools.md) for more details.
 
 ## Results
 

docs/qml/qcl.md

@@ -114,7 +114,7 @@ assert loss.item() < 1e-3
 ```
 
 Qadence offers some convenience functions to implement this training loop with advanced
-logging and metrics track features. You can refer to [this](../qml/qml_tools.md) for more details.
+logging and metrics track features. You can refer to [this tutorial](../qml/ml_tools.md) for more details.
 
 The quantum model is now trained on the training data points. To determine the quality of the results,
 one can check to see how well it fits the function on the test set.