Add warning when importing caffe2 on build without BUILD_CAFFE2=1
Confusing backtraces are issued to users when they run Caffe2 scripts (or tests) on PyTorch builds without Caffe2 enabled through `BUILD_CAFFE2=1`
This PR adds warnings (in more than one place) to return a friendly message for the user, helping them to overcome the problem by themselves
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73770
Approved by: https://github.com/BowenBao, https://github.com/malfet, https://github.com/garymm
