Adding "global", "per_atom", "per_system" features to SimState

[curtischong]

In an effort to properly type the entire library, I'm proposing to add these attributes to the SimState. This way, we don't need to run infer_property_scope (https://github.com/Radical-AI/torch-sim/blob/main/torch_sim/state.py#L496) and we automatically know if a property belongs globally to the entire simstate, or per-atom/system.

This idea is partially inspired by how orb handles features. They use:

"node_features"
"edge_features"
"system_features"

https://github.com/orbital-materials/orb-models/blob/main/orb_models/forcefield/base.py#L65-L67

Maybe if a state is classified as an "md_state" we'll guarantee exactly which features it'll have? Hopefully by solving this problem, we'll also come up with a good solution to represent md_state

[curtischong]

Note: global is actually a reserved keyword. so we'll probably rename it to "shared". #189

[curtischong]

I think I've got an idea of how it'll look like. We'll make these attributes dictionaries in SimState

"node_features"
"edge_features"
"system_features"
"shared_features" (all systems in the simstate have these features)

Then we can add derived classes like MDState. For these derived classes, they store their data inside these attribute dictionaries
HOWEVER, they will have utility getter/setter functions that reads from these attributes.
e.g. if we want to access velocities, we'll use sim_state.velocities(). This way, we can also handle nice edge cases where the velocities may be None at the start of the simulations inside of these utility functions

perhaps important config data should be stored under shared_features.