MaskIntensityD to support user-specified data field

[wyli]

Describe the bug
MaskIntensityD supports to use numpy array as a constructor argument for the mask array,
but one of the common use cases is to dynamically set the mask array from a user-specified data key

MONAI/monai/transforms/intensity/dictionary.py

Lines 470 to 485 in 50c4659

	class MaskIntensityd(MapTransform):
	"""
	Dictionary-based wrapper of :py:class:`monai.transforms.MaskIntensity`.
	Args:
	keys: keys of the corresponding items to be transformed.
	See also: :py:class:`monai.transforms.compose.MapTransform`
	mask_data: if mask data is single channel, apply to evey channel
	of input image. if multiple channels, the channel number must
	match input data. mask_data will be converted to `bool` values
	by `mask_data > 0` before applying transform to input image.
	"""
	def __init__(self, keys: KeysCollection, mask_data: np.ndarray) -> None:
	super().__init__(keys)

Expected behavior
the transform should support:
def __init__(self, keys: KeysCollection, mask_keys: KeysCollection) where data[mask_keys] will be used as the masking array

[Nic-Ma]

Good suggestion. I will update it soon.
Thanks for raising it up.