add HMDB51 and UCF101 datasets as well as prototype for new style video decoding

[pmeier]

Redo of #5335 since I can't reopen it after it was automatically closed.

[facebook-github-bot]

💊 CI failures summary and remediations

As of commit b8a1eac (more details on the Dr. CI page):

---

• 2/2 failures introduced in this PR

---

🕵️ 1 new failure recognized by patterns

The following CI failures do not appear to be due to upstream breakages:

unittest_prototype (1/1)

Step: "Run tests" (full log | diagnosis details | 🔁 rerun)

�[31m=============================== �[31m�[1m1...0.72s�[0m�[31m ===============================�[0m

Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
test/test_prototype_videoutils.py:6: in <module>
    from torchvision.io import _HAS_VIDEO_DECODER, _HAS_VIDEO_OPT, VideoReader
E   ImportError: cannot import name '_HAS_VIDEO_DECODER' from 'torchvision.io' (/home/circleci/project/torchvision/io/__init__.py)�[0m
------ generated xml file: /home/circleci/project/test-results/junit.xml -------
=========================== short test summary info ============================
ERROR test/test_prototype_videoutils.py
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
�[31m=============================== �[31m�[1m1 error�[0m�[31m in 0.72s�[0m�[31m ===============================�[0m


Exited with code exit status 2

---

1 failure not recognized by patterns:

Job	Step	Action
lint_python_and_config	Lint Python code and config files	🔁 rerun

---

This comment was automatically generated by Dr. CI (expand for details).

Please report bugs/suggestions to the (internal) Dr. CI Users group.

Click here to manually regenerate this comment.

[pmeier]

Suggested change

	side_effect=TypeError(
	side_effect=pytest.UsageError(