*******************
Manual Optimization
*******************

For advanced research topics like reinforcement learning, sparse coding, or GAN research, it may be desirable to
manually manage the optimization process, especially when dealing with multiple optimizers at the same time.

In this mode, Lightning will handle only accelerator, precision and strategy logic.
The users are left with ``optimizer.zero_grad()``, gradient accumulation, optimizer toggling, etc..

To manually optimize, do the following:

* Set ``self.automatic_optimization=False`` in your ``LightningModule``'s ``__init__``.
* Use the following functions and call them manually:

  * ``self.optimizers()`` to access your optimizers (one or multiple)
  * ``optimizer.zero_grad()`` to clear the gradients from the previous training step
  * ``self.manual_backward(loss)`` instead of ``loss.backward()``
  * ``optimizer.step()`` to update your model parameters
  * ``self.toggle_optimizer()`` and ``self.untoggle_optimizer()``, or ``self.toggled_optimizer()`` if needed

Here is a minimal example of manual optimization.

.. testcode:: python

    from lightning.pytorch import LightningModule


    class MyModel(LightningModule):
        def __init__(self):
            super().__init__()
            # Important: This property activates manual optimization.
            self.automatic_optimization = False

        def training_step(self, batch, batch_idx):
            opt = self.optimizers()
            opt.zero_grad()
            loss = self.compute_loss(batch)
            self.manual_backward(loss)
            opt.step()

.. tip::
   Be careful where you call ``optimizer.zero_grad()``, or your model won't converge.
   It is good practice to call ``optimizer.zero_grad()`` before ``self.manual_backward(loss)``.


Access your Own Optimizer
=========================

The provided ``optimizer`` is a :class:`~lightning.pytorch.core.optimizer.LightningOptimizer` object wrapping your own optimizer
configured in your :meth:`~lightning.pytorch.core.LightningModule.configure_optimizers`. You can access your own optimizer
with ``optimizer.optimizer``. However, if you use your own optimizer to perform a step, Lightning won't be able to
support accelerators, precision and profiling for you.

.. testcode:: python

   class Model(LightningModule):
       def __init__(self):
           super().__init__()
           self.automatic_optimization = False
           ...

       def training_step(self, batch, batch_idx):
           optimizer = self.optimizers()

           # `optimizer` is a `LightningOptimizer` wrapping the optimizer.
           # To access it, do the following.
           # However, it won't work on TPU, AMP, etc...
           optimizer = optimizer.optimizer
           ...

Gradient Accumulation
=====================

You can accumulate gradients over batches similarly to ``accumulate_grad_batches`` argument in
:ref:`Trainer <trainer>` for automatic optimization. To perform gradient accumulation with one optimizer
after every ``N`` steps, you can do as such.

.. testcode:: python

    def __init__(self):
        super().__init__()
        self.automatic_optimization = False


    def training_step(self, batch, batch_idx):
        opt = self.optimizers()

        # scale losses by 1/N (for N batches of gradient accumulation)
        loss = self.compute_loss(batch) / N
        self.manual_backward(loss)

        # accumulate gradients of N batches
        if (batch_idx + 1) % N == 0:
            opt.step()
            opt.zero_grad()

Gradient Clipping
=================

You can clip optimizer gradients during manual optimization similar to passing the ``gradient_clip_val`` and
``gradient_clip_algorithm`` argument in :ref:`Trainer <trainer>` during automatic optimization.
To perform gradient clipping with one optimizer with manual optimization, you can do as such.

.. testcode:: python

    from lightning.pytorch import LightningModule


    class SimpleModel(LightningModule):
        def __init__(self):
            super().__init__()
            self.automatic_optimization = False

        def training_step(self, batch, batch_idx):
            opt = self.optimizers()

            # compute loss
            loss = self.compute_loss(batch)

            opt.zero_grad()
            self.manual_backward(loss)

            # clip gradients
            self.clip_gradients(opt, gradient_clip_val=0.5, gradient_clip_algorithm="norm")

            opt.step()

.. warning::
   * Note that ``configure_gradient_clipping()`` won't be called in Manual Optimization. Instead consider using ``self. clip_gradients()`` manually like in the example above.


Use Multiple Optimizers (like GANs)
===================================

Here is an example training a simple GAN with multiple optimizers using manual optimization.

.. testcode:: python

    import torch
    from torch import Tensor
    from lightning.pytorch import LightningModule


    class SimpleGAN(LightningModule):
        def __init__(self):
            super().__init__()
            self.G = Generator()
            self.D = Discriminator()

            # Important: This property activates manual optimization.
            self.automatic_optimization = False

        def sample_z(self, n) -> Tensor:
            sample = self._Z.sample((n,))
            return sample

        def sample_G(self, n) -> Tensor:
            z = self.sample_z(n)
            return self.G(z)

        def training_step(self, batch, batch_idx):
            # Implementation follows the PyTorch tutorial:
            # https://pytorch.org/tutorials/beginner/dcgan_faces_tutorial.html
            g_opt, d_opt = self.optimizers()

            X, _ = batch
            batch_size = X.shape[0]

            real_label = torch.ones((batch_size, 1), device=self.device)
            fake_label = torch.zeros((batch_size, 1), device=self.device)

            g_X = self.sample_G(batch_size)

            ##########################
            # Optimize Discriminator #
            ##########################
            d_x = self.D(X)
            errD_real = self.criterion(d_x, real_label)

            d_z = self.D(g_X.detach())
            errD_fake = self.criterion(d_z, fake_label)

            errD = errD_real + errD_fake

            d_opt.zero_grad()
            self.manual_backward(errD)
            d_opt.step()

            ######################
            # Optimize Generator #
            ######################
            d_z = self.D(g_X)
            errG = self.criterion(d_z, real_label)

            g_opt.zero_grad()
            self.manual_backward(errG)
            g_opt.step()

            self.log_dict({"g_loss": errG, "d_loss": errD}, prog_bar=True)

        def configure_optimizers(self):
            g_opt = torch.optim.Adam(self.G.parameters(), lr=1e-5)
            d_opt = torch.optim.Adam(self.D.parameters(), lr=1e-5)
            return g_opt, d_opt

Learning Rate Scheduling
========================

Every optimizer you use can be paired with any
`Learning Rate Scheduler <https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate>`_. Please see the
documentation of :meth:`~lightning.pytorch.core.LightningModule.configure_optimizers` for all the available options

You can call ``lr_scheduler.step()`` at arbitrary intervals.
Use ``self.lr_schedulers()`` in  your :class:`~lightning.pytorch.core.LightningModule` to access any learning rate schedulers
defined in your :meth:`~lightning.pytorch.core.LightningModule.configure_optimizers`.

.. warning::
   * ``lr_scheduler.step()`` can be called at arbitrary intervals by the user in case of manual optimization, or by Lightning if ``"interval"`` is defined in :meth:`~lightning.pytorch.core.LightningModule.configure_optimizers` in case of automatic optimization.
   * Note that the ``lr_scheduler_config`` keys, such as ``"frequency"`` and ``"interval"``, will be ignored even if they are provided in
     your :meth:`~lightning.pytorch.core.LightningModule.configure_optimizers` during manual optimization.

Here is an example calling ``lr_scheduler.step()`` every step.

.. testcode:: python

    # step every batch
    def __init__(self):
        super().__init__()
        self.automatic_optimization = False

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=1e-3)
        scheduler = torch.optim.lr_scheduler.StepLR(optimizer, step_size=10, gamma=0.1)
        return [optimizer], [scheduler]

    def training_step(self, batch, batch_idx):
        # do forward, backward, and optimization
        ...

        # single scheduler
        sch = self.lr_schedulers()
        sch.step()

        # multiple schedulers
        sch1, sch2 = self.lr_schedulers()
        sch1.step()
        sch2.step()

If you want to call ``lr_scheduler.step()`` every ``N`` steps/epochs, do the following.

.. testcode:: python

    def __init__(self):
        super().__init__()
        self.automatic_optimization = False

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=1e-3)
        scheduler = torch.optim.lr_scheduler.StepLR(optimizer, step_size=10, gamma=0.1)
        return [optimizer], [scheduler]


    def training_step(self, batch, batch_idx):
        # do forward, backward, and optimization
        ...

        sch = self.lr_schedulers()

        # step every N batches
        if (batch_idx + 1) % N == 0:
            sch.step()

        # step every N epochs
        if self.trainer.is_last_batch and (self.trainer.current_epoch + 1) % N == 0:
            sch.step()

If you want to call schedulers that require a metric value after each epoch, consider doing the following:

.. testcode::

    def __init__(self):
        super().__init__()
        self.automatic_optimization = False

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=1e-3)
        scheduler = torch.optim.lr_scheduler.ReduceLROnPlateau(optimizer, mode='min', factor=0.1, patience=10)
        return [optimizer], [scheduler]

    def on_train_epoch_end(self):
        sch = self.lr_schedulers()

        sch.step(self.trainer.callback_metrics["loss"])

.. note::
    :meth:`~lightning.pytorch.core.LightningModule.configure_optimizers` supports 6 different ways to define and return
    optimizers and learning rate schedulers. Regardless of the way you define them, `self.optimizers()` will always return
    either a single optimizer if you defined a single optimizer, or a list of optimizers if you defined multiple
    optimizers. The same applies to the `self.lr_schedulers()` method, which will return a single scheduler
    if you defined a single scheduler, or a list of schedulers if you defined multiple schedulers


Optimizer Steps at Different Frequencies
========================================

In manual optimization, you are free to ``step()`` one optimizer more often than another one.
For example, here we step the optimizer for the *discriminator* weights twice as often as the optimizer for the *generator*.

.. testcode:: python

    # Alternating schedule for optimizer steps (e.g. GANs)
    def training_step(self, batch, batch_idx):
        g_opt, d_opt = self.optimizers()
        ...

        # update discriminator every other step
        d_opt.zero_grad()
        self.manual_backward(errD)
        if (batch_idx + 1) % 2 == 0:
            d_opt.step()

        ...

        # update generator every step
        g_opt.zero_grad()
        self.manual_backward(errG)
        g_opt.step()


Use Closure for LBFGS-like Optimizers
=====================================

It is a good practice to provide the optimizer with a closure function that performs a ``forward``, ``zero_grad`` and
``backward`` of your model. It is optional for most optimizers, but makes your code compatible if you switch to an
optimizer which requires a closure, such as :class:`~torch.optim.LBFGS`.

See `the PyTorch docs <https://pytorch.org/docs/stable/optim.html#optimizer-step-closure>`_ for more about the closure.

Here is an example using a closure function.

.. testcode:: python

    def __init__(self):
        super().__init__()
        self.automatic_optimization = False


    def configure_optimizers(self):
        return torch.optim.LBFGS(...)


    def training_step(self, batch, batch_idx):
        opt = self.optimizers()

        def closure():
            loss = self.compute_loss(batch)
            opt.zero_grad()
            self.manual_backward(loss)
            return loss

        opt.step(closure=closure)

.. warning::
   The :class:`~torch.optim.LBFGS` optimizer is not supported for AMP or DeepSpeed.
