Summary:
`np.str` is removed from numpy 1.20.0. It was an alias to builtin `str` and it's safe to do the replacement.
The whole changes is mechanical, generated using the following onliner:
```
fbgr -sl 'np\.str\b' | xargs perl -pi -e 's,\bnp\.str\b,str,g'
```
Test Plan: sandcastle
Differential Revision: D46586144
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103931
Approved by: https://github.com/huydhn