Reserve memory option in ApproxKNN

[rusty1s]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 89.25%. Comparing base (ec4cde3) to head (1b9f032).

Files	Patch %	Lines
torch_geometric/nn/pool/knn.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9046      +/-   ##
==========================================
- Coverage   89.26%   89.25%   -0.02%     
==========================================
  Files         468      468              
  Lines       30041    30049       +8     
==========================================
+ Hits        26815    26819       +4     
- Misses       3226     3230       +4     

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