Make finding unused model parameters optional (#19515)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19515
This is still done by default, but can now be disabled by specifying
`find_unused_parameters=False`. There are use cases where finding
unused parameters results in erroneous behavior, because a subset of
model parameters is used *outside* the `forward` function. One can
argue that doing this is not a good idea, but we should not break
existing use cases without an escape hatch. This configuration
parameter is that escape hatch.
Reviewed By: bddppq
Differential Revision: D15016381
fbshipit-source-id: f2f86b60771b3801ab52776e62b5fd6748ddeed0
