Consolidate mypy tests and args (#50631)

Summary:
This PR helps with https://github.com/pytorch/pytorch/issues/50513 by reducing the complexity of our `mypy` test suite and making it easier to reproduce on the command line. Previously, to reproduce how `mypy` was actually run on tracked source files (ignoring the doctest typechecking) in CI, you technically needed to run 9 different commands with various arguments:
```
$ mypy --cache-dir=.mypy_cache/normal --check-untyped-defs --follow-imports silent
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/module_list.py
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/namedtuple.py
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/opt_size.py
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/size.py
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/tensor_copy.py
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/torch_cuda_random.py
$ mypy --cache-dir=.mypy_cache/examples --follow-imports silent --check-untyped-defs test/type_hint_tests/torch_optim.py
$ mypy --cache-dir=.mypy_cache/strict --config mypy-strict.ini
```
Now you only have to run 2 much simpler commands:
```
$ mypy
$ mypy --config mypy-strict.ini
```
One reason this is useful is because it will make it easier to integrate PyTorch's `mypy` setup into editors (remaining work on this to be done in a followup PR).

Also, as shown in the test plan, this also reduces the time it takes to run `test/test_type_hints.py` incrementally, by reducing the number of times `mypy` is invoked while still checking the same set of files with the same configs.

(Because this PR merges `test_type_hint_examples` (added in https://github.com/pytorch/pytorch/issues/34595) into `test_run_mypy` (added in https://github.com/pytorch/pytorch/issues/36584), I've added some people involved in those PRs as reviewers, in case there's a specific reason they weren't combined in the first place.)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/50631

Test Plan:
Run this twice (the first time is to warm the cache):
```
$ python test/test_type_hints.py -v
```

- *Before:*
  ```
  test_doc_examples (__main__.TestTypeHints)
  Run documentation examples through mypy. ... ok
  test_run_mypy (__main__.TestTypeHints)
  Runs mypy over all files specified in mypy.ini ... ok
  test_run_mypy_strict (__main__.TestTypeHints)
  Runs mypy over all files specified in mypy-strict.ini ... ok
  test_type_hint_examples (__main__.TestTypeHints)
  Runs mypy over all the test examples present in ... ok

  ----------------------------------------------------------------------
  Ran 4 tests in 5.090s

  OK
  ```
  You can also just run `mypy` to see how many files it checks:
  ```
  $ mypy --cache-dir=.mypy_cache/normal --check-untyped-defs --follow-imports silent
  Success: no issues found in 1192 source files
  ```
- *After:*
  ```
  test_doc_examples (__main__.TestTypeHints)
  Run documentation examples through mypy. ... ok
  test_run_mypy (__main__.TestTypeHints)
  Runs mypy over all files specified in mypy.ini ... ok
  test_run_mypy_strict (__main__.TestTypeHints)
  Runs mypy over all files specified in mypy-strict.ini ... ok

  ----------------------------------------------------------------------
  Ran 3 tests in 2.404s

  OK
  ```
  Now `mypy` checks 7 more files, which is the number in `test/type_hint_tests`:
  ```
  $ mypy
  Success: no issues found in 1199 source files
  ```

Reviewed By: zou3519

Differential Revision: D25932660

Pulled By: samestep

fbshipit-source-id: 26c6f00f338e7b44954e5ed89522ce24e2fdc5f0

mypy-strict.ini

@@ -7,6 +7,7 @@
 [mypy]
 python_version = 3.6
 
+cache_dir = .mypy_cache/strict
 strict_optional = True
 show_column_numbers = True
 warn_no_return = True