enable to_mxfp8 cast for DTensor

Summary:

1. add a test for casting a DTensor to mxfp8
2. make the test pass:
  a. remove addition of epsilon, it's not supported in DTensor world but
  we also don't need it anymore since we are no longer using `log2`
  anywhere.
  b. replace `<<` with `torch.bitwise_left_shift` and `>>` with
  `torch.bitwise_right_shift`.  The short versions are silently broken
  for DTensor inputs, but the verbose versions work.
3. set up the wiring for testing mxfp8 with TP on a toy model. Note that
   making this work is split for the next PR, as this PR got too large.

Test Plan:

```bash
./test/prototype/mx_formats/test_dtensor.sh
./test/float8/test_dtensor.sh
pytest test/prototype/mx_formats/
```

Reviewers:

Subscribers:

Tasks:

Tags:

ghstack-source-id: e054ed4
ghstack-comment-id: 2993264092
Pull Request resolved: #2420

Author: vkuzo