Summary:
This PR adds a local [`mypy` plugin](https://mypy.readthedocs.io/en/stable/extending_mypy.html#extending-mypy-using-plugins) that warns if you accidentally run `mypy` using a version that doesn't match [the version we install for CI](6045663f39/.circleci/docker/common/install_conda.sh (L117)), since this trips people up sometimes when `mypy` gives errors in some versions (see https://github.com/pytorch/pytorch/issues/51513) but not others.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51799
Test Plan:
To check that this doesn't break our `mypy` test(s) when you have the correct version installed:
```
python test/test_type_hints.py
```
To check that this does indeed warn when you have an incorrect `mypy` version installed, switch to a different version (e.g. 0.782), and run the above command or either of these:
```
mypy
mypy --config-file=mypy-strict.ini
```
You should get the following message on stderr:
```
You are using mypy version 0.782, which is not supported
in the PyTorch repo. Please switch to mypy version 0.770.
For example, if you installed mypy via pip, run this:
pip install mypy==0.770
Or if you installed mypy via conda, run this:
conda install -c conda-forge mypy=0.770
```
Reviewed By: janeyx99
Differential Revision: D26282010
Pulled By: samestep
fbshipit-source-id: 7b423020d0529700dea8972b27afa2d7068e1b12
