[functorch] Added cross batching rule (pytorch/functorch#144)
* Added cross batching rule
Description:
- Added cross batching rule
- Updated tests
- Fixed issue in test/common_utils.py with repeated number 3 in dims. Set it to 4 and 2 to avoid special value for cross op
* Covered batch_size=3 corner case
* Added batch_size = 3 specific test for cross op
