Is your feature request related to a problem? Please describe.
Currently, our ConfigParser can't set new nested ID, for example "key1#2#key2" if even "key1" doesn't exist:
https://github.com/Project-MONAI/MONAI/blob/dev/monai/bundle/config_parser.py#L159

Would be nice to support new nested ID.