Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/3327
**Context**
1. [Issue 2621](https://github.com/facebookresearch/faiss/issues/2621) discuss inconsistency between OnDiskInvertedList and InvertedList. OnDiskInvertedList is supposed to handle disk based multiple Index Shards. Thus, we should name it differently when merging invls from index shard.
2. [Issue 2876](https://github.com/facebookresearch/faiss/issues/2876) provides usecase of shifting ids when merging invls from different shards.
**In this diff**,
1. To address #1 above, I renamed the merge_from function to merge_from_multiple without touching merge_from base class.
why so? To continue to allow merge invl from one index to ondiskinvl from other index.
2. To address #2 above, I have added support of shift_ids in merge_from_multiple to shift ids from different shards. This can be used when each shard has same set of ids but different data. This is not recommended if id is already unique across shards.
Reviewed By: mdouze
Differential Revision: D55482518
fbshipit-source-id: 95470c7449160488d2b45b024d134cbc037a2083
Michael Norris
Kumar Saurabh Arora
Ben Mann
Lucas Hosseini