improve libtorch install docs with GPU note (#17299)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/15702
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17299
Differential Revision: D14149712
Pulled By: soumith
fbshipit-source-id: 5b83110bb00e4d4dad04c1f293c2b52e41711f11
