update index: dtype constraints changed in ATen (#321)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/321

Previously reported index [issue](https://github.com/pytorch/pytorch/issues/107698) related to input/out type conversion was fixed in ATen, by requiring out tensor to have the same dtype than input tensor.
This diffs updates the portable index kernel, to stay Dtype-compliant.

Reviewed By: larryliu0820

Differential Revision: D49218705

fbshipit-source-id: 0aa16a8ffd666c69b9386bd6b2a8c1e23398872c