Allow forward functions with single output to return Variable (#23803)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23803
Custom `forward()` can return a `Variable` in case of single outputs instead of returning a `variable_list` of size 1.
Test Plan: Modified tests involving single output forward functions.
Reviewed By: ezyang
Differential Revision: D16673857
Pulled By: ezyang
fbshipit-source-id: c96d9473b48ad99e6736a68d334b333a917498b7
