Add experimental version of leiden_communities

[eriknw]

This function is not yet defined or implemented in NetworkX, and there is high probability the API may change once added to NetworkX. For now, this is "best effort" and simply mimics PLC leiden.

[eriknw]

It appears that theta parameter is not used, so I removed it for now.

First, it is ignored in the C->Python API here:
https://github.com/rapidsai/cugraph/blob/1f1cace91e9b97c11dc6d57026360c3a0a919676/cpp/src/c_api/leiden.cpp#L128-L141

Second, it is ignored in the refinement step here (done in rapidsai/cugraph#3990):
https://github.com/rapidsai/cugraph/blob/1f1cace91e9b97c11dc6d57026360c3a0a919676/cpp/src/community/detail/refine_impl.cuh#L107-L124

[eriknw]

Begin adding to networkx here: networkx/networkx#7743

[rlratzel]

As @eriknw mentioned in the description, this is still experimental and is not exposed to users by default (because it is not currently available in NetworkX) and is low risk, and also may be useful to have in 24.12. I'm fine merging this into 24.12 for those reasons.

[rlratzel]

re: 24.12 vs. 25.02 - on second thought, @eriknw pointed out the potential for the API to change which would make 24.12 incompatible with future NX versions.