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Add soft error to index and add method to file set

Summary:
## This diff
file set can use native `set` here, but file set have  index and add method that native set don't have.
So adding the soft error to see if there is any useage

This diff also add bit_or which could be used to migrate, since native set also have this.

## Why not add  index and add method for native set
We should better keep the same api as python set

Reviewed By: Will-MingLun-Li

Differential Revision: D72076363

fbshipit-source-id: 2d6e29822dd6466edba4af949ab54eff792acb3f