Adds upload_sonic_decoder.py, which derives the SONIC PD gains (kp/kd) and the
residual action_scale from Unitree motor physics (armature + target bandwidth +
per-motor effort), and bakes them plus default_angles and the neutral idle token
into the nvidia/GEAR-SONIC decoder ONNX metadata, uploading the result to
lerobot/sonic_decoder. The runtime then loads all constants from that checkpoint,
so this motor-physics derivation lives here rather than in the deploy path.
Co-authored-by: Cursor <cursoragent@cursor.com>
Martino Russi