Clarification on `scatter` utilities

[ffl096]

TopoModelX contains some scatter functions in utils.scatter, that -- according to the module docstring -- are adapted from torch_scatter. Is there a specific reason why we ship our own implementation? torch_scatter is a dependency already.

If yes, we should document that reason and how the build-in implementation differs from torch_scatter (which, for me, is not at all obvious from looking at the code). Otherwise remove it and use the existing implementation in torch_scatter everywhere.