Initial vmap + NT support with unbind fallback (#106786)
PoC demonstrating vmap + NT based on the [design doc](https://docs.google.com/document/d/1dVVk6TOqz93PLTIneU2T3xaxCs9qZ0MaJyCvOAp_bC0). This PR:
* Allows `BatchedTensorImpl`s to contain NTs
* Introduces a `BatchedNestedTensor` dispatch key for NT-specific batching rules
* Provides a batching rule fallback that unbinds the NTs -> performs computation on constituent -> rebinds results into NT
Restrictions:
* Only supports one level of vmap
* Only supports vmapping over dim=0 for NTs
* For operations with mixed NT / dense inputs, support is also limited to dim=0 for the dense inputs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106786
Approved by: https://github.com/zou3519
