Split nn.MultiHeadAttention into Module + functional (#20415)
Summary:
Moving functions from torch/nn/modules/activation.py to torch/nn/functional.py. For functions not implemented (_get_input_buffer and _set_input_buffer), a TODO is added.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20415
Differential Revision: D15318078
Pulled By: jamarshon
fbshipit-source-id: 5ca698e2913821442cf8609cc61ac8190496a3c6
