Better type Module.apply signature - PR discussion #4779

Following up on #2086 and this recent SO question How to type hint flax.linen.Module.apply's output correctly?.

I would like to add a PR based on my answer and add two typing.overload signatures that make it more clear when to expect a tuple return value - following the contribution guidelines I am opening this issue first.

Based on #2086, the docstring and

I am certain that only mutable=False never returns a tuple, otherwise it will always be a tuple.