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