map_classes_to_indices warns pending operations

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https://github.com/Project-MONAI/MONAI/blob/1a55ba5423d04d2ef7ac19356ccabc4c7906f577/monai/transforms/utils.py#L332

before computing coordinates from the input image, raise a warning if there are pending operations in it.