BERT Proxy Bench: Multiple sentence batch, Layer Norm, GELU (#3833)

Summary:
Pull Request resolved: #3833

Previously could only handle one sentence. Now there are two parameters: maxSequenceLength and batchSize. Also added layer norm and GELU.

Reviewed By: yinghai

Differential Revision: D18542972

fbshipit-source-id: 087d35d055b33b313a4fffef21793fa0970b7427

Author: mjanderson09