Update MultiHeadAttention docstring (#49950)
Summary:
Fixes MultiHeadAttention docstring.
Currently, https://pytorch.org/docs/stable/generated/torch.nn.MultiheadAttention.html#torch.nn.MultiheadAttention
is
<img width="648" alt="Screen Shot 2020-12-29 at 21 06 43" src="https://user-images.githubusercontent.com/2459423/103311124-cd10cc00-4a19-11eb-89c9-0ee261364963.png">
and with the fix will be
<img width="648" alt="Screen Shot 2020-12-29 at 22 41 35" src="https://user-images.githubusercontent.com/2459423/103315838-0dc31200-4a27-11eb-82e2-ca8f13d713a1.png">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49950
Reviewed By: mrshenli
Differential Revision: D25732573
Pulled By: zhangguanheng66
fbshipit-source-id: b362f3f617ab26b0dd25c3a0a7d4117e522e620c
