Remove hard Caffe2 dependency for TensorBoard (#24295)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/24175 and https://github.com/pytorch/pytorch/issues/15618
We should not be importing caffe2 (and dependencies like future, etc) unless needed within `torch.utils.tensorboard`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24295
Reviewed By: NarineK
Differential Revision: D16797594
Pulled By: orionr
fbshipit-source-id: 984935e2121b62ea1b87a9de33af18ec45b7837b
