Remove caffe2::ShareData (#15418)

Remove caffe2::ShareData (#15418)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15418

Previously we are using Resize + ShareData.
Instead, we'll create a function on Tensor that clones itself with same storage.

Suppose we want `t` to `ShareData` with `t0`, Previous:
```
Tensor t(dims, CPU);
t.Resize(t0.sizes());
t.ShareData(t0);
```
Now:
```
Tensor t = t0.Alias();
```

Reviewed By: dzhulgakov

Differential Revision: D13507609

fbshipit-source-id: 6e4275d02f4c3356cbce91127f1b01111dc86b9f