Add rst entry for nn.MultiheadAttention (#19346)
Summary:
Fix #19259 by adding the missing `autoclass` entry for `nn.MultiheadAttention` from [here](https://github.com/pytorch/pytorch/blob/master/torch/nn/modules/activation.py#L676)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19346
Differential Revision: D14971426
Pulled By: soumith
fbshipit-source-id: ceaaa8ea4618c38fa2bff139e7fa0d6c9ea193ea
