Neural Architecture Search with Retiarii (Alpha)
================================================

*This is a pre-release, its interfaces may subject to minor changes. The roadmap of this feature is: experimental in V2.0 -> alpha version in V2.1 -> beta version in V2.2 -> official release in V2.3. Feel free to give us your comments and suggestions.*

`Retiarii <https://www.usenix.org/system/files/osdi20-zhang_quanlu.pdf>`__ is a new framework to support neural architecture search and hyper-parameter tuning. It allows users to express various search space with high flexibility, to reuse many SOTA search algorithms, and to leverage system level optimizations to speed up the search process. This framework provides the following new user experiences.

* Search space can be expressed directly in user model code. A tuning space can be expressed during defining a model.
* Neural architecture candidates and hyper-parameter candidates are more friendly supported in an experiment.
* The experiment can be launched directly from python code.

.. Note:: `Our previous NAS framework <../Overview.rst>`__ is still supported for now, but will be migrated to Retiarii framework in V2.3.

.. contents::

There are mainly two crucial components for a neural architecture search task, namely,

* Model search space that defines the set of models to explore.
* A proper strategy as the method to explore this search space.
* A model evaluator that reports the performance of a given model.

.. note:: Currently, PyTorch is the only supported framework by Retiarii, and we have only tested with **PyTorch 1.6 and 1.7**. This documentation assumes PyTorch context but it should also apply to other frameworks, that is in our future plan.

Define your Model Space
-----------------------

Model space is defined by users to express a set of models that users want to explore, which contains potentially good-performing models. In this framework, a model space is defined with two parts: a base model and possible mutations on the base model.

Define Base Model
^^^^^^^^^^^^^^^^^

Defining a base model is almost the same as defining a PyTorch (or TensorFlow) model. Usually, you only need to replace the code ``import torch.nn as nn`` with ``import nni.retiarii.nn.pytorch as nn`` to use our wrapped PyTorch modules.

Below is a very simple example of defining a base model, it is almost the same as defining a PyTorch model.

.. code-block:: python

  import torch.nn.functional as F
  import nni.retiarii.nn.pytorch as nn

  @basic_unit
  class BasicBlock(nn.Module):
    def __init__(self, const):
      self.const = const
    def forward(self, x):
      return x + self.const

  class ConvPool(nn.Module):
    def __init__(self):
      super().__init__()
      self.conv = nn.Conv2d(32, 1, 5)  # possibly mutate this conv
      self.pool = nn.MaxPool2d(kernel_size=2)
    def forward(self, x):
      return self.pool(self.conv(x))

  class Model(nn.Module):
    def __init__(self):
      super().__init__()
      self.convpool = ConvPool()
      self.mymodule = BasicBlock(2.)
    def forward(self, x):
      return F.relu(self.convpool(self.mymodule(x)))

The above example also shows how to use ``@basic_unit``. ``@basic_unit`` is decorated on a user-defined module to tell Retiarii that there will be no mutation within this module, Retiarii can treat it as a basic unit (i.e., as a blackbox). It is useful when (1) users want to mutate the initialization parameters of this module, or (2) Retiarii fails to parse this module due to complex control flow (e.g., ``for``, ``while``). More detailed description of ``@basic_unit`` can be found `here <./Advanced.rst>`__.

Users can refer to :githublink:`Darts base model <test/retiarii_test/darts/darts_model.py>` and :githublink:`Mnasnet base model <examples/nas/multi-trial/mnasnet/base_mnasnet.py>` for more complicated examples.

Define Model Mutations
^^^^^^^^^^^^^^^^^^^^^^

A base model is only one concrete model not a model space. We provide APIs and primitives for users to express how the base model can be mutated, i.e., a model space which includes many models.

We provide some APIs as shown below for users to easily express possible mutations after defining a base model. The APIs can be used just like PyTorch module. This approach is also called inline mutations.

* ``nn.LayerChoice``. It allows users to put several candidate operations (e.g., PyTorch modules), one of them is chosen in each explored model. Note that if the candidate is a user-defined module, it should be decorated as a `basic unit <./Advanced.rst>`__ with ``@basic_unit``. In the following example, ``ops.PoolBN`` and ``ops.SepConv`` should be decorated.

  .. code-block:: python

    # import nni.retiarii.nn.pytorch as nn
    # declared in `__init__` method
    self.layer = nn.LayerChoice([
      ops.PoolBN('max', channels, 3, stride, 1),
      ops.SepConv(channels, channels, 3, stride, 1),
      nn.Identity()
    ]))
    # invoked in `forward` method
    out = self.layer(x)

* ``nn.InputChoice``. It is mainly for choosing (or trying) different connections. It takes several tensors and chooses ``n_chosen`` tensors from them.

  .. code-block:: python

    # import nni.retiarii.nn.pytorch as nn
    # declared in `__init__` method
    self.input_switch = nn.InputChoice(n_chosen=1)
    # invoked in `forward` method, choose one from the three
    out = self.input_switch([tensor1, tensor2, tensor3])

* ``nn.ValueChoice``. It is for choosing one value from some candidate values. It can only be used as input argument of basic units, that is, modules in ``nni.retiarii.nn.pytorch`` and user-defined modules decorated with ``@basic_unit``.

  .. code-block:: python

    # import nni.retiarii.nn.pytorch as nn
    # used in `__init__` method
    self.conv = nn.Conv2d(XX, XX, kernel_size=nn.ValueChoice([1, 3, 5])
    self.op = MyOp(nn.ValueChoice([0, 1]), nn.ValueChoice([-1, 1]))

All the APIs have an optional argument called ``label``, mutations with the same label will share the same choice. A typical example is,

  .. code-block:: python

    self.net = nn.Sequential(
        nn.Linear(10, nn.ValueChoice([32, 64, 128], label='hidden_dim'),
        nn.Linear(nn.ValueChoice([32, 64, 128], label='hidden_dim'), 3)
    )

Detailed API description and usage can be found `here <./ApiReference.rst>`__\. Example of using these APIs can be found in :githublink:`Darts base model <test/retiarii_test/darts/darts_model.py>`. We are actively enriching the set of inline mutations, to make it easier to express a new search space.

If the inline mutation APIs are not enough for your scenario, you can refer to `defining model space using mutators <./Advanced.rst#express-mutations-with-mutators>`__ to write more complex model spaces.

Explore the Defined Model Space
-------------------------------

There are basically two exploration approaches: (1) search by evaluating each sampled model independently and (2) one-shot weight-sharing based search. We demonstrate the first approach below in this tutorial. Users can refer to `here <./OneshotTrainer.rst>`__ for the second approach.

Users can choose a proper search strategy to explore the model space, and use a chosen or user-defined model evaluator to evaluate the performance of each sampled model.

Choose a search strategy
^^^^^^^^^^^^^^^^^^^^^^^^

Retiarii currently supports the following search strategies:

* Grid search: enumerate all the possible models defined in the space.
* Random: randomly pick the models from search space.
* Regularized evolution: a genetic algorithm that explores the space based on inheritance and mutation.

Choose (i.e., instantiate) a search strategy is very easy. An example is as follows,

.. code-block:: python

  import nni.retiarii.strategy as strategy

  search_strategy = strategy.Random(dedup=True)  # dedup=False if deduplication is not wanted

Detailed descriptions and usages of available strategies can be found `here <./ApiReference.rst>`__ .

Choose or write a model evaluator
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In the NAS process, the search strategy repeatedly generates new models. A model evaluator is for training and validating each generated model. The obtained performance of a generated model is collected and sent to search strategy for generating better models.

The model evaluator should correctly identify the use case of the model and the optimization goal. For example, on a classification task, an <input, label> dataset is needed, the loss function could be cross entropy and the optimized metric could be accuracy. On a regression task, the optimized metric could be mean-squared-error.

In the context of PyTorch, Retiarii has provided two built-in model evaluators, designed for simple use cases: classification and regression. These two evaluators are built upon the awesome library PyTorch-Lightning.

An example here creates a simple evaluator that runs on MNIST dataset, trains for 10 epochs, and reports its validation accuracy.

.. code-block:: python

  import nni.retiarii.evaluator.pytorch.lightning as pl
  from nni.retiarii import serialize
  from torchvision import transforms

  transform = serialize(transforms.Compose, [serialize(transforms.ToTensor()), serialize(transforms.Normalize, (0.1307,), (0.3081,))])
  train_dataset = serialize(MNIST, root='data/mnist', train=True, download=True, transform=transform)
  test_dataset = serialize(MNIST, root='data/mnist', train=False, download=True, transform=transform)
  evaluator = pl.Classification(train_dataloader=pl.DataLoader(train_dataset, batch_size=100),
                                val_dataloaders=pl.DataLoader(test_dataset, batch_size=100),
                                max_epochs=10)

As the model evaluator is running in another process (possibly in some remote machines), the defined evaluator, along with all its parameters, needs to be correctly serialized. For example, users should use the dataloader that has been already wrapped as a serializable class defined in ``nni.retiarii.evaluator.pytorch.lightning``. For the arguments used in dataloader, recursive serialization needs to be done, until the arguments are simple types like int, str, float.

Detailed descriptions and usages of model evaluators can be found `here <./ApiReference.rst>`__ .

If the built-in model evaluators do not meet your requirement, or you already wrote the training code and just want to use it, you can follow `the guide to write a new evaluator <./WriteTrainer.rst>`__ .

.. note:: In case you want to run the model evaluator locally for debug purpose, you can directly run the evaluator via ``evaluator._execute(Net)`` (note that it has to be ``Net``, not ``Net()``). However, this API is currently internal and subject to change.

.. warning:: Mutations on the parameters of model evaluator (known as hyper-parameter tuning) is currently not supported but will be supported in the future.

.. warning:: To use PyTorch-lightning with Retiarii, currently you need to install PyTorch-lightning v1.1.x (v1.2 is not supported).

Launch an Experiment
--------------------

After all the above are prepared, it is time to start an experiment to do the model search. An example is shown below.

.. code-block:: python

  exp = RetiariiExperiment(base_model, trainer, None, simple_strategy)
  exp_config = RetiariiExeConfig('local')
  exp_config.experiment_name = 'mnasnet_search'
  exp_config.trial_concurrency = 2
  exp_config.max_trial_number = 10
  exp_config.training_service.use_active_gpu = False
  exp.run(exp_config, 8081)

The complete code of a simple MNIST example can be found :githublink:`here <examples/nas/multi-trial/mnist/search.py>`.

**Local Debug Mode**: When running an experiment, it is easy to get some trivial errors in trial code, such as shape mismatch, undefined variable. To quickly fix these kinds of errors, we provide local debug mode which locally applies mutators once and runs only that generated model. To use local debug mode, users can simply invoke the API `debug_mutated_model(base_model, trainer, applied_mutators)`.

Visualize the Experiment
------------------------

Users can visualize their experiment in the same way as visualizing a normal hyper-parameter tuning experiment. For example, open ``localhost::8081`` in your browser, 8081 is the port that you set in ``exp.run``. Please refer to `here <../../Tutorial/WebUI.rst>`__ for details.