Add warning when importing caffe2 on build without BUILD_CAFFE2=1
Confusing backtraces are issues to user when they try to run tests or actual scripts using Caffe2 on a pytorch build without Caffe2 enabled through BUILD_CAFFE2=1
This PR adds a warning in more than one place to return a friendly message for the user
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73770
Approved by: https://github.com/BowenBao, https://github.com/malfet, https://github.com/garymm
