Speed-up "advanced" indexing operations (#13420)

Speed-up "advanced" indexing operations (#13420)

Summary:
This speeds-up "advanced" indexing (indexing a tensor by a tensor)
on CPU and GPU. There's still a bunch of work to do, including
speeding up indexing by a byte (boolean) mask and speeding up the derivative
calculation for advanced indexing.

Here's some speed comparisons to indexing on master using a little [benchmark script](https://gist.github.com/colesbury/c369db72aad594e5e032c8fda557d909) with 16 OpenMP threads and on a P100. The test cases are listed as (input shape -> output shape).

| Test case             | CPU (old vs. new)   | CUDA (old vs. new)     |
|-----------------------|---------------------|------------------------|
| 1024x1024 -> 512x1024 | 225 us vs. **57 us**  | 297 us vs. **47 us** |
| 1024x1024 -> 1024x512 | 208 us vs. **153 us** | 335 us vs. **54 us** |
| 50x50 -> 20000x50     | 617 us vs. **77 us**  | 239 us vs. **54 us** |
| 50x50 -> 50x20000     | 575 us vs. **236 us** | 262 us vs. **58 us** |
| 2x5x10 -> 10          | 65 us  vs. **18 us**  | 612 us vs. **93 us** |

See #11647
Pull Request resolved: https://github.com/pytorch/pytorch/pull/13420

Reviewed By: soumith

Differential Revision: D13088936

Pulled By: colesbury

fbshipit-source-id: 0a5c2ee9aa54e15f96d06692d1694c3b24b924e2