TDMPC unable to run only on robot state - proposed fix

[genemerewether]

TDMPC cannot function without image features or environment state, only because of this line:

lerobot/lerobot/common/policies/tdmpc/modeling_tdmpc.py

Line 343 in f24030d

"observation.image" if self.config.image_features else "observation.environment_state"

The rest of the logic allows running with only robot state. Could we change this line to else "observation.state"? Tested locally and works great with that change.