Add entry point to tell NetworkX about nx-cugraph without importing it.

[eriknw]

This allows NetworkX docstrings to be updated (among other things).

This will have a companion PR in NetworkX. We still need to determine (and agree) on the dict returned by this entry point, and NetworkX doesn't need to use everything I have here. We should probably add a string for "description" that gives a very short description of the backend, and maybe "url" or "homepage" or whatever so online docs can have links.

Here's how to use the entry point (Python >= 3.10) after installing it:

In [1]: from importlib.metadata import entry_points

In [2]: items = entry_points(group="networkx.plugin_info")

In [3]: [plugin] = items

In [4]: plugin.load()()
Out[4]:
{'backend_name': 'cugraph',
 'project': 'nx-cugraph',
 'package': 'nx_cugraph',
 'functions': {'betweenness_centrality',
  'edge_betweenness_centrality',
  'louvain_communities'},
 'extra_docstrings': {'betweenness_centrality': '`weight` parameter is not yet supported.',
  'edge_betweenness_centrality': '`weight` parameter is not yet supported.',
  'louvain_communities': '`threshold` and `seed` parameters are currently ignored.'},
 'extra_parameters': {'louvain_communities': {'max_level': 'Upper limit of the number of macro-iterations.'}}}

CC @rlratzel @betochimas

[eriknw]

Companion networkx PR is up: networkx/networkx#6911

[eriknw]

Alternatively, we could organize info like:

        "functions": {
            # BEGIN: functions
            "betweenness_centrality": {
                "extra_docstring": "`weight` parameter is not yet supported.",
            },
            "edge_betweenness_centrality" : {
                "extra_docstring": "`weight` parameter is not yet supported.",
            },
            "louvain_communities": {
                "extra_docstring": "louvain_communities": "`threshold` and `seed` parameters are currently ignored.",
                "extra_parameters": {
                    "max_level : int, optional": "Upper limit of the number of macro-iterations.",
                },
            },
            # END: functions
        },

[rlratzel]

Looked good, nothing jumped out. I had a couple of questions though.

[rlratzel]

Is this dead code we can remove?

[eriknw]

It is future code, yet to be born

[eriknw]

@rlratzel mentioned we should update louvain to handle isolated nodes as done in #3886. I'll go ahead and make this change in this PR.

[BradReesWork]

/merge