Only import MultiheadAttention when doing typechecking
Only import MultiheadAttention when doing typechecking

Summary: This might create dependency loops in some cases and we don't use it otherwise.

Reviewed By: myleott, dianaml0

Differential Revision: D29521387

fbshipit-source-id: b4c27426a5965cd864a0c7b353082756099fead5

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq