chore(async): move async related code to its directory at top level (#2003)

* chore(async): move async related code to its directory at top level

* chore(style): apply pre-commit to renamed headers

* test(async): fix async imports

* docs(async): update async headers doc

tests/async_inference/test_helpers.py

@@ -19,8 +19,7 @@ import time
 import numpy as np
 import torch
 
-from lerobot.configs.types import FeatureType, PolicyFeature
-from lerobot.scripts.server.helpers import (
+from lerobot.async_inference.helpers import (
     FPSTracker,
     TimedAction,
     TimedObservation,
@@ -30,6 +29,7 @@ from lerobot.scripts.server.helpers import (
     raw_observation_to_observation,
     resize_robot_observation_image,
 )
+from lerobot.configs.types import FeatureType, PolicyFeature
 
 # ---------------------------------------------------------------------
 # FPSTracker