Port test/test_datasets.py to use pytest

[yiwen-song]

Following the discussion in #3945, we should also port test/test_dataset to use pytest.

In this PR, we replace all the self.assertXYZ with python assert.

TODO: Replace the self.subTest() in these files and finally remove the unittest.TestCase inheritance.

[NicolasHug]

Thanks @sallysyw , contrags on your first torchvision PR!
This looks great, there are just 2 assertions that need a minor change. I'll approve anyway, feel free to self-merge once the changes are addressed

[NicolasHug]

Indeed the next step will be to fully remove the use of unittest.TestCase and thus remove the subTest parts, which I think is fine.

It'd be nice to change the test_all_configs decorator into a pytest parametrization. @pmeier do you foresee any major complication in doing that?

[github-actions]

Hey @sallysyw!

You merged this PR, but no labels were added.

[pmeier]

It'd be nice to change the test_all_configs decorator into a pytest parametrization. @pmeier do you foresee any major complication in doing that?

IIRC, you can't use pytest.mark.parametrize on the test methods of a unittest.TestCase. So we would also need to change that class. But then, we are currently using the TestCase.setUpClass method

vision/test/datasets_utils.py

Lines 387 to 392 in b29ed34

	@classmethod
	def setUpClass(cls):
	cls._verify_required_public_class_attributes()
	cls._populate_private_class_attributes()
	cls._process_optional_public_class_attributes()
	super().setUpClass()

which we would have to emulate another way.

[NicolasHug]

IIRC, you can't use pytest.mark.parametrize on the test methods of a unittest.TestCase

That's correct, and getting rid of unittest altogether is our end goal here :). Regarding the setup, we'll probably just have to rename it into a setup_class method instead: https://docs.pytest.org/en/6.2.x/xunit_setup.html#class-level-setup-teardown