Add OfflineTensor
Summary: 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: ChunliF, zrphercule
Differential Revision: D18187208
fbshipit-source-id: 57c70f6f9897a5fc66580c81295db108acd03862
