MapLabelValue for identity mapping

[wyli]

Is your feature request related to a problem? Please describe.
the current label mapping transform raises an error when it's configured to do identity mapping:

MONAI/monai/transforms/utility/array.py

Lines 1222 to 1223 in 0247672

	if all(o == z for o, z in zip(orig_labels, target_labels)):
	raise ValueError("orig_labels and target_labels are exactly the same, should be different to map.")

this logic is too restrictive when building flexible mapping pipelines, would be great to remove the checking.