Add XLA / TPU device type, backend type and type id (#16763)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16763
Replicate the easy bits in https://github.com/pytorch/pytorch/pull/15153 with TPU / XLA instead of MSNPU. Also don't initialize the storage for XLA tensors for now.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16585
Reviewed By: ezyang
Differential Revision: D13912118
Pulled By: gchanan
fbshipit-source-id: 4889177e2478768fb281ed075b71146d1d850bd9
