store LSUN cache in root directory rather than in cwd

[pmeier]

Follow-up from and blocked by #3454 (comment).

[pmeier]

This only came up, because the tests are generating a lot of these cache files. If you think this should stay the way it is we can simply remove them afterwards:

vision/test/test_datasets.py

Lines 959 to 971 in 9846569

	@contextlib.contextmanager
	def create_dataset(
	self,
	*args, **kwargs
	):
	with super().create_dataset(*args, **kwargs) as output:
	yield output
	# Currently datasets.LSUN caches the keys in the current directory rather than in the root directory. Thus,
	# this creates a number of unique _cache_* files in the current directory that will not be removed together
	# with the temporary directory
	for file in os.listdir(os.getcwd()):
	if file.startswith("_cache_"):
	os.remove(file)

Thus, I'm closing this. Feel free to re-open in case we want to change the caching.