feat(policies): add autoregressive VLAs with tokenization PiFast (#2734)

src/lerobot/utils/constants.py

@@ -28,6 +28,8 @@ OBS_LANGUAGE_TOKENS = OBS_LANGUAGE + ".tokens"
 OBS_LANGUAGE_ATTENTION_MASK = OBS_LANGUAGE + ".attention_mask"
 
 ACTION = "action"
+ACTION_TOKENS = ACTION + ".tokens"
+ACTION_TOKEN_MASK = ACTION + ".token_mask"
 REWARD = "next.reward"
 TRUNCATED = "next.truncated"
 DONE = "next.done"

src/lerobot/utils/import_utils.py

@@ -63,6 +63,7 @@ def is_package_available(pkg_name: str, return_version: bool = False) -> tuple[b
 
 _transformers_available = is_package_available("transformers")
 _peft_available = is_package_available("peft")
+_scipy_available = is_package_available("scipy")
 
 
 def make_device_from_device_class(config: ChoiceRegistry) -> Any: