[functorch] Simplify BatchedTensorImpl (#84642)
Three changes:
- deleted an unused constructor
- simplified an implementation from the old days when a
BatchedTensorImpl could have multiple bdims
- added a comment about getKeysToPropagateToWrapper
Differential Revision: [D39315049](https://our.internmc.facebook.com/intern/diff/D39315049)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84642
Approved by: https://github.com/samdow
