Dealing with Module Arguments

Introduction

In Linen we can define Module arguments either as dataclass attributes or as arguments to methods (usually __call__). Typically the distinction is clear:

  • Completely fixed properties, such as the choice of kernel initializer or number of output features, are hyperparameters and should be defined as dataclass attributes. Typically two Module instances with different hyperparamaters cannot share in a meaningful way.

  • Dynamic properties, such as input data and top-level “mode switches” like train=True/False, should be passed as arguments to __call__ or another method.

Some cases are however less clear cut. Take for example the Dropout module. We have a number of clear hyperparameters:

  1. The dropout rate

  2. The axes for which a dropout mask is generated

And some clear call time arguments:

  1. The input that should be masked using dropout

  2. The (optional) rng used to sample the random mask

There is however one property that is ambiguous – the deterministic property in a Dropout module.

If deterministic is True no dropout mask is sampled. This is typically used during model evaluation. However, if we pass eval=True or train=False to a top-level Module. The deterministic argument needs to be applied everywhere and the boolean argument needs to be passed down to all the layers that might use Dropout. If instead deterministic is a dataclass attribute, we might do the following:

from functools import partial
from flax import linen as nn

class ResidualModel(nn.Module):
  drop_rate: float

  @nn.compact
  def __call__(self, x, *, train):
    dropout = partial(nn.Dropout, rate=self.drop_rate, deterministic=not train)
    for i in range(10):
      x += ResidualBlock(dropout=dropout, ...)(x)

It makes sense to pass determinstic to the constructor here because this way we can pass the dropout template to the sub-modules. Now the sub-module no longer needs to take care of train vs eval mode and can simply use the dropout argument. Note that because the dropout layer can only be constructed in the sub-module we can only partially apply deterministic to the constructor but not to __call__.

However, if deterministic is a dataclass attribute we run into trouble when using the setup pattern. We would want to write our module code like this:

class SomeModule(nn.Module):
  drop_rate: float

  def setup(self):
    self.dropout = nn.Dropout(rate=self.drop_rate)

  @nn.compact
  def __call__(self, x, *, train):
    # ...
    x = self.dropout(x, deterministic=not train)
    # ...

But, as defined above, deterministic would be an attribute, so this doesn’t work. Here it makes sense to pass deterministic during __call__ because it depends on the train argument.

Solution

We can support both use cases described before by allowing certain properties to be passed as dataclass attributes or as method argument (but not both!). This can be implemented as follows:

class MyDropout(nn.Module):
  drop_rate: float
  deterministic: Optional[bool] = None

  @nn.compact
  def __call__(self, x, deterministic=None):
    deterministic = nn.merge_param('deterministic', self.deterministic, deterministic)
    # ...

In this example nn.merge_param will ensure that either self.deterministic or deterministic is set but not both. An error is raised if both values are None or both values are not None. This avoids confusing behavior where 2 different parts of the code set the same parameter and one is overruled by the other. It also avoids a default value which would probably cause either the train step or eval step of a training procedure to be broken by default.

Functional Core and flax.nn

The old NN api and functional core define functions rather than classes. Therefore, there is no clear distinction between hyper parameters and call time arguments. The only way to pre-determine the hyper parameters is by using partial. On the upside, there are no ambiguous cases where method arguments could also be attributes.