Add __main__ guards to fx tests (#154715)

This PR is part of a series attempting to re-submit #134592 as smaller PRs.

In fx tests:

- Add and use a common raise_on_run_directly method for when a user runs a test file directly which should not be run this way. Print the file which the user should have run.
- Raise a RuntimeError on tests which have been disabled (not run)
- Remove any remaining uses of "unittest.main()""

Pull Request resolved: https://github.com/pytorch/pytorch/pull/154715
Approved by: https://github.com/Skylion007

torch/testing/_internal/common_utils.py

@@ -902,6 +902,11 @@ def prof_callable(callable, *args, **kwargs):
 
     return callable(*args, **kwargs)
 
+def raise_on_run_directly(file_to_call):
+    raise RuntimeError("This test file is not meant to be run directly, "
+                       f"use:\n\n\tpython {file_to_call} TESTNAME\n\n"
+                       "instead.")
+
 def prof_func_call(*args, **kwargs):
     return prof_callable(func_call, *args, **kwargs)