[functorch] Added Index.Tensor batching rule (pytorch/functorch#123)
* handled some cases of index.Tensor
* fixed merge errors
* Added batching rules for index, both cases are batched
* fix some issues
* handled some cases of index.Tensor
* fixed merge errors
* Added batching rules for index, both cases are batched
* fix some issues
* fix tests
* handled some cases of index.Tensor
* fixed merge errors
* fixed tests
