Add faiss-based KNNIndex classes

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #7842 (dab47e2) into master (bd20485) will decrease coverage by 0.64%.
Report is 1 commits behind head on master.
The diff coverage is 86.27%.

❗ Current head dab47e2 differs from pull request most recent head 5742ada. Consider uploading reports for the commit 5742ada to get more accurate results

@@            Coverage Diff             @@
##           master    #7842      +/-   ##
==========================================
- Coverage   90.44%   89.80%   -0.64%     
==========================================
  Files         455      456       +1     
  Lines       26335    26383      +48     
==========================================
- Hits        23819    23694     -125     
- Misses       2516     2689     +173     

Files Changed	Coverage Δ
torch_geometric/nn/pool/knn.py	86.00% <86.00%> (ø)
torch_geometric/nn/pool/__init__.py	73.33% <100.00%> (+0.60%)	⬆️

... and 31 files with indirect coverage changes

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