Speed up DataTypeToTypeMeta (#66113)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66113
For a benchmark compiled in opt-mode in which the lookup items were shuffled and then the items were looked up round-robin fashion 10M times (for a total of 140M lookups) compiled in opt-mode we see:
```
Function Container Time (ms) Multiplier
TypeMetaToDataType if-chain 233 1x
TypeMetaToDataType std::vector 795 3.41x
TypeMetaToDataType std::map 1566 6.72x
TypeMetaToDataType std::unordered_map 2136 9.17x
DataTypeToTypeMeta switch 102 1x
DataTypeToTypeMeta std::vector 666 6.53x
DataTypeToTypeMeta std::map 1212 11.9x
DataTypeToTypeMeta std::unordered_map 1539 15.1x
DataTypeToTypeMeta folly::F14FastMap 1789 17.5x
```
From this, we draw two conclusions:
1. Using a complex container like `std::map` is worse than using a simple vector lookup here (there aren't enough items for the Big-O to assert itself).
2. Using any container at all is a mistake. (Unless we pull in more exotic reasoning like invalidating the code cache or preventing inlining.)
Test Plan: Sandcastle
Reviewed By: dzhulgakov
Differential Revision: D31375117
fbshipit-source-id: 0b310c6c2e94080d125c82fb7c2b43ab869adbcb
