Allow RNN hidden_size to be 0 (#70556)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/56767.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70556
Reviewed By: ngimel
Differential Revision: D33455156
Pulled By: jbschlosser
fbshipit-source-id: 5dc57b09d7beb6ae81dfabc318e87c109bb4e6ae
