[cuGraph] Fix test_gat_conv_equality for new edge_attr requirement in CuGraphGATConv

[drivanov]

Updates test_gat_conv_equality to explicitly handle the new required edge_attr argument in CuGraphGATConv.forward.
The test now parametrizes edge features and correctly passes either a real edge feature tensor or an empty (num_edges, 0) tensor for non-bipartite graphs, matching cuGraph kernel expectations.
All forward and backward equality checks now pass across all parameter combinations.

Without these changes I see that 96 subtests of test_cugraph_gat_conv.py::test_gat_conv_equality are failing with the following error message message:

    def _call_impl(self, *args, **kwargs):
        forward_call = (self._slow_forward if torch._C._get_tracing_state() else self.forward)
        # If we don't have any hooks, we want to skip the rest of the logic in
        # this function, and just call forward.
        if not (self._backward_hooks or self._backward_pre_hooks or self._forward_hooks or self._forward_pre_hooks
                or _global_backward_pre_hooks or _global_backward_hooks
                or _global_forward_hooks or _global_forward_pre_hooks):
>           return forward_call(*args, **kwargs)
E           TypeError: CuGraphGATConv.forward() missing 1 required positional argument: 'edge_attr'

/usr/local/lib/python3.12/dist-packages/torch/nn/modules/module.py:1787: TypeError

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.09%. Comparing base (c211214) to head (d543155).
⚠️ Report is 143 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10564      +/-   ##
==========================================
- Coverage   86.11%   85.09%   -1.02%     
==========================================
  Files         496      510      +14     
  Lines       33655    35964    +2309     
==========================================
+ Hits        28981    30604    +1623     
- Misses       4674     5360     +686     

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