Add an AssertError check back to MultiheadAttention module (#20492)
Summary:
Fix a typo in the doc.
Add an AssertError check back to MultiheadAttention module
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20492
Differential Revision: D15349008
Pulled By: cpuhrsch
fbshipit-source-id: 2d898345f03787c713e537673613a748ad826b34
