[dist_optim] add warning to distributed optimizer (#50630)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50630
Add a warning log to distributed optimizer, to warn user the optimizer
is created without TorchScript support.
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D25932777
Pulled By: wanchaol
fbshipit-source-id: 8db3b98bdd27fc04c5a3b8d910b028c0c37f138d
