Fix confusing "does not have GPU support" warning message (#30721)
Summary:
Many people who use caffe2 are confused about "does not have GPU support" warning message.
https://github.com/facebookresearch/video-nonlocal-net/issues/6
facebookarchive/caffe2#346
facebookarchive/caffe2#1634
facebookarchive/caffe2#197
Many none GPU reasons can cause this warning message. It is better to give the error info.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30721
Differential Revision: D19697413
Pulled By: ezyang
fbshipit-source-id: bd24b7c814e7e677352068b9e9f77a68de080159
