Inconsistent python dialect registration

[stellaraccident]

The dialect registration functions for MHLO are of the form mhlo.register_mhlo_dialect(context) and chlo.register_chlo_dialect(context).

We don't have an upstream entry for this in the style guide (but maybe should); however, everyone else has converged on mhlo.register_dialect(context). Would be nice to remove the redundant "mhlo" and "chlo". Since already in use, can deprecate the originals.