add tests for perspective start- / endpoints

[pmeier]

Addresses #7215 (comment). There is no general issue with the tests, but rather missing sample inputs for perspective_bounding_box. We only tested with coefficients=..., but left startpoints=endpoints=None:

vision/test/prototype_transforms_kernel_infos.py

Lines 1200 to 1204 in 0aed832

	def sample_inputs_perspective_bounding_box():
	for bounding_box_loader in make_bounding_box_loaders():
	yield ArgsKwargs(
	bounding_box_loader, bounding_box_loader.format, None, None, coefficients=_PERSPECTIVE_COEFFS[0]
	)

This means we never hit never hit this branch

vision/torchvision/prototype/transforms/functional/_geometry.py

Lines 1266 to 1267 in 0aed832

	elif startpoints is not None and endpoints is not None:
	return _get_perspective_coeffs(startpoints, endpoints)

Meaning, the test suite never noticed that we passed garbage there. I've added tests for passing startpoints and endpoints explicitly. Without the fix in this PR, the bounding box test would now fail.

cc @vfdev-5 @bjuncek

[NicolasHug]

Thanks Philip