Speeds up fast-path for 1D tensors (#22756)
Summary:
Using PMCTest (https://www.agner.org/optimize/) to measure
TensorIterator construction, this results in ~600 fewer instructions
retired (~300 fewer cycles) for constructing TensorIterator on a 1D
tensor. (Should be roughly ~100 ns, but it's hard to measure that
precisely end-to-end).
```
Before:
Clock Core cyc Instruct Uops L1D Miss
5082 2768 5690 7644 3
After:
Clock Core cyc Instruct Uops L1D Miss
4518 2437 5109 6992 0
```
Note that Instruct is reliable, Core cyc is a little noisy, and Clock
is a little more noisy.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22756
Differential Revision: D16207777
Pulled By: VitalyFedyunin
fbshipit-source-id: bcc453a90472d9951a1c123bcb1b7a243fde70ac
