[functorch] Refactored BatchedTensorImpl to have single bdim and level (pytorch/functorch#239)
* Refactored BatchedTensorImpl to have single bdim and level
Description:
- Refactored BatchedTensorImpl to have single bdim and level
- Made uniform signature to receive (level, bdim) as args. Method makeBatched is impacted in many files.
- Maybe I could adopt (bdim, level) but both were defined.
Fixes pytorch/functorch#86
* Updated code according to the review
