… in unit testing (#96444)

Set environment variable
```
PYTORCH_TEST_DO_NOT_USE_PYTEST=1
```
to not use pytest in pytorch unit testing.

This change is related to some recent changes, e.g. #96210, #96016, #95844, #95659, that enabled the use of pytest in many test modules. Those test modules were testing normally before, but failed immediately after pytest is used. Sample stacktraces are:

```python
root@8e3168a83ee2:/opt/pytorch/pytorch# python test/run_test.py -v -i test_optim -- -v --save-xml
Ignoring disabled issues:  []
/opt/pytorch/pytorch/test/run_test.py:1225: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
  if torch.version.cuda is not None and LooseVersion(torch.version.cuda) >= "11.6":
Selected tests:
 test_optim
parallel (file granularity) tests:
 test_optim
serial (file granularity) tests:

Ignoring disabled issues:  []
Ignoring disabled issues:  []
Running test_optim ... [2023-03-09 12:51:59.358110]
Executing ['/usr/local/bin/python', '-bb', 'test_optim.py', '-v', '--save-xml', '-v', '--use-pytest', '-vv', '-rfEX', '-x', '--reruns=2'] ... [2023-03-09 12:51:59.358810]

Test results will be stored in test-reports/python-pytest/test_optim/test_optim-5e41643c8bac8ace.xml
Traceback (most recent call last):
  File "/opt/pytorch/pytorch/test/test_optim.py", line 4581, in <module>
    run_tests()
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_utils.py", line 796, in run_tests
    exit_code = pytest.main(args=pytest_args)
  File "/usr/local/lib/python3.10/site-packages/_pytest/config/__init__.py", line 148, in main
    config = _prepareconfig(args, plugins)
  File "/usr/local/lib/python3.10/site-packages/_pytest/config/__init__.py", line 329, in _prepareconfig
    config = pluginmanager.hook.pytest_cmdline_parse(
  File "/usr/local/lib/python3.10/site-packages/pluggy/_hooks.py", line 265, in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
  File "/usr/local/lib/python3.10/site-packages/pluggy/_manager.py", line 80, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/usr/local/lib/python3.10/site-packages/pluggy/_callers.py", line 55, in _multicall
    gen.send(outcome)
  File "/usr/local/lib/python3.10/site-packages/_pytest/helpconfig.py", line 103, in pytest_cmdline_parse
    config: Config = outcome.get_result()
  File "/usr/local/lib/python3.10/site-packages/pluggy/_result.py", line 60, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/usr/local/lib/python3.10/site-packages/pluggy/_callers.py", line 39, in _multicall
    res = hook_impl.function(*args)
  File "/usr/local/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1060, in pytest_cmdline_parse
    self.parse(args)
  File "/usr/local/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1348, in parse
    self._preparse(args, addopts=addopts)
  File "/usr/local/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1231, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "/usr/local/lib/python3.10/site-packages/pluggy/_manager.py", line 287, in load_setuptools_entrypoints
    plugin = ep.load()
  File "/usr/local/lib/python3.10/importlib/metadata/__init__.py", line 171, in load
    module = import_module(match.group('module'))
  File "/usr/local/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "/usr/local/lib/python3.10/site-packages/_pytest/assertion/rewrite.py", line 168, in exec_module
    exec(co, module.__dict__)
  File "/usr/local/lib/python3.10/site-packages/xdist/looponfail.py", line 16, in <module>
    import execnet
  File "/usr/local/lib/python3.10/site-packages/execnet/__init__.py", line 14, in <module>
    from .gateway_base import DataFormatError
  File "/usr/local/lib/python3.10/site-packages/execnet/gateway_base.py", line 1138, in <module>
    FLOAT_FORMAT_SIZE = struct.calcsize(FLOAT_FORMAT)
BytesWarning: Comparison between bytes and string
FINISHED PRINTING LOG FILE of test_optim (/opt/pytorch/pytorch/test/test-reports/test_optim_1pnlesrz.log)

test_optim failed!
Traceback (most recent call last):
  File "/opt/pytorch/pytorch/test/run_test.py", line 1428, in <module>
    main()
  File "/opt/pytorch/pytorch/test/run_test.py", line 1386, in main
    raise RuntimeError(
RuntimeError: test_optim failed!

Tip: You can keep running tests even on failure by passing --keep-going to run_test.py.
If running on CI, add the 'keep-going' label to your PR and rerun your jobs.
```

I'd like to propose this option that allows users to use the good old python unit test way instead of pytest to run their testing in CI.

Pull Request resolved: #96444
Approved by: https://github.com/malfet

Implements --subprocess flag for pytest, which previously only worked with unittest

Pretty much all the tests in the custom handler list use --subprocess
Pull Request resolved: pytorch/pytorch#96210
Approved by: https://github.com/huydhn

Implements --subprocess flag for pytest, which previously only worked with unittest

Pretty much all the tests in the custom handler list use --subprocess
Pull Request resolved: pytorch/pytorch#96210
Approved by: https://github.com/huydhn

Implements --subprocess flag for pytest, which previously only worked with unittest

Pretty much all the tests in the custom handler list use --subprocess
Pull Request resolved: pytorch#96210
Approved by: https://github.com/huydhn