Remove StateDict from torchsim integration

[orionarcher]

Summary

torch_sim.typing.StateDict has been removed in TorchSim/torch-sim#487. This causes an ImportError when loading the NequIP torchsim integration:

ImportError: cannot import name 'StateDict' from 'torch_sim.typing'

This PR updates nequip/integrations/torchsim.py to:

• Remove the from torch_sim.typing import StateDict import
• Change forward(self, state: ts.SimState | StateDict) → forward(self, state: ts.SimState)
• Replace the isinstance ternary that fell back to dict→SimState conversion with a direct sim_state = state assignment

Test plan

• Verify NequIP torchsim integration tests pass against torch-sim main (post 🌟 [FEATURE] Training NequIP on Local Spectroscopic Properties (e.g., NMR parameters) for Specific Atoms Only #487)

🤖 Generated with Claude Code

[cw-tan]

Thanks, I changed it to merge to develop branch. In general, PRs to nequip should be to the develop branch. Will merge when tests pass.

[orionarcher]

Thanks @cw-tan!