Add type hints to methods in interaction_net.py

Summary

The methods in neural_lam/interaction_net.py currently lack type annotations. mypy is already configured in pre-commit but cannot fully validate this module without proper type hints.

Methods affected

• InteractionNet.forward
• InteractionNet.message
• InteractionNet.aggregate
• SplitMLPs.forward

Proposed Fix

Add torch.Tensor, Optional[torch.Tensor], Tuple and Union type hints to all method signatures and return types.

Why This Matters

• mypy is already set up in .pre-commit-config.yaml
• Type hints improve contributor experience and catch bugs early
• Consistent with type hints already added in metrics.py (see #446)