Allow TransformerEncoder and TransformerDecoder to accept 0-dim batch sized tensors...

Allow TransformerEncoder and TransformerDecoder to accept 0-dim batch sized tensors. (#62800)

Summary:
This issue fixes a part of https://github.com/pytorch/pytorch/issues/12013, which is summarized concretely in  https://github.com/pytorch/pytorch/issues/38115.

This PR allows TransformerEncoder and Decoder (alongwith the inner `Layer` classes) to accept inputs with 0-dimensional batch sizes.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/62800

Reviewed By: VitalyFedyunin

Differential Revision: D30303240

Pulled By: jbschlosser

fbshipit-source-id: 8f8082a6f2a9f9d7ce0b22a942d286d5db62bd12