fix knn_merge_parts for inner product

[qwertyforce]

By default, when merging the results of sharded multigpu index, knn_merge_parts keeps K smallest values. But inner product is not a distance, it is a measure of similarity. Therefore results are wrong for IP.
In this PR we are adding an additional overload, that receives an argument select_min, we keep backward compatibility and add a new test

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[divyegala]

Hi @qwertyforce, thank you for the PR! I think this change will severely affect the binary size of cuVS, can you please provide a before-after measurement? Alternatively, we could run an element-wise operation to negate the inner product distances before running the merge kernel.

[qwertyforce]

Hi! i ran ./build.sh libcuvs --allgpuarch --no-nvtx -n (GCC 13, CUDA 12.8)
main vs patched
libcuvs.so increased from 504MB to 512MB
knn_merge_parts.cu.o increased from 8MB to 16MB

[divyegala]

Thanks @qwertyforce , in that case can we please pursue the alternative of running a negation for inner product?

[qwertyforce]

Yes, will try to implement and benchmark it