Add OfflineTensor (#28855)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28855
Resubmit:
OfflineTensor will be a shell to just carry the shape and dtype. No data will be stored. This should help us plumb through the onnxifi process.
Test Plan:
```
buck test caffe2/caffe2/fb/opt:onnxifi_with_offline_tensor_test
```
Reviewed By: ipiszy, ChunliF
Differential Revision: D18212824
fbshipit-source-id: 5c8aaed2ef11d719dfa2a2901875efd66806ea56
