Fix TestTypeHints.test_doc_examples (#56388)
Summary:
https://github.com/pytorch/pytorch/issues/54268 removed `test_run_mypy` since now we're running `mypy` as its own job in GitHub Actions, but previously we used this `set_cwd` context manager in that test to ensure that we picked up the `mypy` config correctly. However, for some reason, we have not been doing that in `test_doc_examples`, which has been succeeding in CI for a while despite being broken.
Specifically, [`run_test.py` changes the working directory to `test/` before running test files](https://github.com/pytorch/pytorch/blob/48aaea3359cffe7982ce24b8742c7a1f4b456a92/test/run_test.py#L534-L535), which is contrary to [what `CONTRIBUTING.md` instructs developers to do](https://github.com/pytorch/pytorch/blob/48aaea3359cffe7982ce24b8742c7a1f4b456a92/CONTRIBUTING.md#python-unit-testing). As a result, in CI, `test/test_type_hints.py` has been passing in CI, but if you run it locally from the root of the repo, this you get this error:
```
F
======================================================================
FAIL: test_doc_examples (__main__.TestTypeHints)
Run documentation examples through mypy.
----------------------------------------------------------------------
Traceback (most recent call last):
File "test/test_type_hints.py", line 127, in test_doc_examples
self.fail(f"mypy failed:\n{stdout}")
AssertionError: mypy failed:
test/generated_type_hints_smoketest.py:851: error: Name 'tensor' is not defined [name-defined]
test/generated_type_hints_smoketest.py:853: error: Name 'tensor' is not defined [name-defined]
Found 2 errors in 1 file (checked 1 source file)
----------------------------------------------------------------------
Ran 1 test in 1.416s
FAILED (failures=1)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56388
Test Plan:
Before this PR, the first of the following two commands should fail (since that is essentially what is run in CI), but the second should fail:
```
python test/run_test.py -i test_type_hints
python test/test_type_hints.py
```
After this PR, both commands should succeed.
Reviewed By: driazati
Differential Revision: D27860173
Pulled By: samestep
fbshipit-source-id: efb82fffd7ccb04d0331824b40bdef7bbc319c98
