PyTorch/Caffe2 tensor interop in Python (#17190)

PyTorch/Caffe2 tensor interop in Python (#17190)

Summary:
Because of two separate python extensions with different pybind
instances I have to go through void* conversion. Since it's hidden from
user, it's fine.

New APIs added on C2 side:
- workspace.FetchTorch('blob')
- workspace.Workspace.current.blobs['blob'].to_torch()
- workspace.FeedBlob('blob', pytorch_tensor)

Works on CPU an GPU.

The only glitches are with resizing because of variable/tensor split.
But data sharing works properly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17190

Reviewed By: ezyang

Differential Revision: D14163882

Pulled By: dzhulgakov

fbshipit-source-id: d18e5b8fcae026f393c842a1149e972515732de2