persistent dataset default `cache_dir`

[wyli]

Is your feature request related to a problem? Please describe.

MONAI/monai/data/dataset.py

Lines 136 to 142 in 45c6cf6

	def __init__(
	self,
	data: Sequence,
	transform: Union[Sequence[Callable], Callable],
	cache_dir: Optional[Union[Path, str]] = None,
	hash_func: Callable[..., bytes] = pickle_hashing,
	) -> None:

the default value of cache_dir is None, which means no caching. this is somewhat unexpected for a caching based dataset.

Describe the solution you'd like
remove the default value so that the user has to provide a location for the cache files