[PyTorch][Static Runtime] Add & use "small array" for ProcessedNodeInputs (#67935)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67935
Rationale should be documented in code comments. In short, we
can avoid heap-allocating arrays of input indexes for operators with 5
or fewer inputs, at the cost of a tag bit check on access.
ghstack-source-id: 143429112
Test Plan:
Patched d1jang's D32181666, which prints static runtime memory usage.
Previous diff, local:
```
I1105 12:17:36.459688 866763 PyTorchStaticRuntimePredictor.cpp:82] memory turnover after creating an instance of StaticRuntime: 354208
```
This diff, local:
```
I1105 12:48:35.820663 1066520 PyTorchStaticRuntimePredictor.cpp:82] memory turnover after creating an instance of StaticRuntime: 338064
```
4.5% savings (16144 bytes)
Ran 10 repetitions of CMF local_ro with core pinning: P467095603. This diff is perf neutral compared to the previous diff.
Reviewed By: hlu1
Differential Revision: D32216573
fbshipit-source-id: d18483db255f75f1d90e610ecded7727c6ffe65c
