[jit] Speed up saving in case of many classes (#44589)
Summary:
There's an annoying O(N^2) in module export logic that makes saving some of the models (if they have many classes) take eternity.
I'm not super familiar with this code to properly untangle the deps and make it a pure hash lookup. So I just added a side lookup table for raw pointers. It's still quadratic, but it's O(num_classes^2) instead of O(num_classes * num_references) which already gives huge savings.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44589
Test Plan:
Tested with one of the offending models - just loading a saving a Torchscript file:
```
Before:
load 1.9239683151245117
save 165.74712467193604
After:
load 1.9409027099609375
save 1.4711427688598633
```
Reviewed By: suo
Differential Revision: D23675278
Pulled By: dzhulgakov
fbshipit-source-id: 8f3fa7730941085ea20d9255b49a149ac1bf64fe
