Recursively find tensors in DDP module output (#19360)

Recursively find tensors in DDP module output (#19360)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19360

We'll return the output object verbatim since it is a freeform object.
We need to find any tensors in this object, though, because we need to
figure out which parameters were used during this forward pass, to
ensure we short circuit reduction for any unused parameters.

Before this commit only lists were handled and the functionality went
untested. This commit adds support for dicts and recursive structures,
and also adds a test case.

Closes #19354.

Reviewed By: mrshenli

Differential Revision: D14978016

fbshipit-source-id: 4bb6999520871fb6a9e4561608afa64d55f4f3a8