Add Policy.from_config() method to match documented API

[samet-akcay]

Summary

The documentation (PR #201) describes Policy.from_config(config) as the primary/recommended way to instantiate policies, but this method doesn't exist yet in the codebase. Currently only Model.from_config() exists.

Current State

# What users must do today (2 steps)
config = ACTConfig(hidden_dim=512, chunk_size=100, ...)
model = ACTModel.from_config(config)
policy = ACT(model=model)

Desired State

# What users should be able to do (1 step)
config = ACTConfig(hidden_dim=512, chunk_size=100, ...)
policy = ACT.from_config(config)

Implementation

The implementation is trivial - add FromConfig mixin to Policy classes:

class FromConfig:
    """Mixin that enables Policy.from_config(config) instantiation."""
    model_type: type
    
    @classmethod
    def from_config(cls, config: Config, **kwargs):
        model = cls.model_type.from_config(config)
        return cls(model=model, **kwargs)

Then update each policy:

class ACT(FromConfig, FromCheckpoint, Export, Policy):
    model_type = ACTModel
    # ... rest unchanged

or alternatively, base Policy class could also inherit FromConfig mixin, so it would be supported by all of the policies by default.

Affected Policies

• ACT
• Diffusion
• GROOT
• Pi0
• SmolVLA

Related

• PR docs: comprehensive documentation overhaul and application README improvements #201 - Documentation that describes this API pattern