[Fix] Improve dim_size handling in SetTransformerAggregation to prevent CUDA crash

[KAVYANSHTYAGI]

This PR improves the robustness of SetTransformerAggregation by:

• Automatically setting dim_size = index.max() + 1 if dim_size is not provided.
• Raising a clear error if index.max() >= dim_size to avoid CUDA crashes during evaluation.

This is helpful especially for datasets like PPI where data.batch may be missing. It replaces hard-to-debug GPU errors with clear and early validation.