Add note that cuda quantization is not supported (#27829)
Summary:
People get confused with partial support otherwise: https://github.com/pytorch/pytorch/issues/27811 #27729
Suggestions on where else put warnings are welcomed (probably in tutorials - cc SethHWeidman )
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27829
Differential Revision: D17910931
Pulled By: dzhulgakov
fbshipit-source-id: 37a169a4bef01b94be59fe62a8f641c3ec5e9b7c
