[FEA]: Update Louvain implementation for isolated vertices

[jnke2016]

Is this a new feature, an improvement, or a change to existing functionality?

New Feature

How would you describe the priority of this feature request

Low (would be nice)

Please provide a clear description of problem this feature solves

The Louvain implementation computes the isolated vertices but this is no longer necessary since the graph creation now support isolated vertices as input.

Describe your ideal solution

Update the Louvain implementation to no longer compute isolated vertices and remove FIXME

Describe any alternatives you have considered

No response

Additional context

No response

Code of Conduct

• [x] I agree to follow cuGraph's Code of Conduct
• [x] I have searched the open feature requests and have found no duplicates for this feature request