Add a microbenchmark for LSTM elementwise portion (#42901)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/42901
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D23079714
Pulled By: bertmaher
fbshipit-source-id: 28f8c3b5019ee898e82e64a0a674da1b4736d252
