feat(eval): batch_size=auto + faster env loading

- batch_size=0 (default) auto-tunes based on CPU cores, capped by
  n_episodes and 64. Removes the need for users to guess the right
  value. The old batch_size > n_episodes error is replaced by silently
  clamping to n_episodes.
- _LazyAsyncVectorEnv accepts pre-computed spaces so only one temp env
  is created per suite (not per task). For libero_spatial (10 tasks)
  this avoids 9 redundant LiberoEnv instantiations during env setup.

Made-with: Cursor

src/lerobot/configs/default.py

@@ -65,21 +65,27 @@ class WandBConfig:
 class EvalConfig:
     n_episodes: int = 50
     # `batch_size` specifies the number of environments to use in a gym.vector.VectorEnv.
-    batch_size: int = 50
+    # Set to 0 for auto-tuning based on available CPU cores and n_episodes.
+    batch_size: int = 0
     # `use_async_envs` specifies whether to use asynchronous environments (multiprocessing).
     # Defaults to True; automatically downgraded to SyncVectorEnv when batch_size=1.
     use_async_envs: bool = True
 
     def __post_init__(self) -> None:
+        if self.batch_size == 0:
+            self.batch_size = self._auto_batch_size()
         if self.batch_size > self.n_episodes:
-            raise ValueError(
-                "The eval batch size is greater than the number of eval episodes "
-                f"({self.batch_size} > {self.n_episodes}). As a result, {self.batch_size} "
-                f"eval environments will be instantiated, but only {self.n_episodes} will be used. "
-                "This might significantly slow down evaluation. To fix this, you should update your command "
-                f"to increase the number of episodes to match the batch size (e.g. `eval.n_episodes={self.batch_size}`), "
-                f"or lower the batch size (e.g. `eval.batch_size={self.n_episodes}`)."
-            )
+            self.batch_size = self.n_episodes
+
+    def _auto_batch_size(self) -> int:
+        """Pick batch_size based on CPU cores, capped by n_episodes."""
+        import math
+        import os
+
+        cpu_cores = os.cpu_count() or 4
+        # Each async env worker needs ~1 core; leave headroom for main process + inference.
+        by_cpu = max(1, math.floor(cpu_cores * 0.7))
+        return min(by_cpu, self.n_episodes, 64)
 
 
 @dataclass

src/lerobot/envs/libero.py

@@ -413,17 +413,25 @@ class _LazyAsyncVectorEnv:
     AsyncVectorEnv on first reset(), keeping peak process count = n_envs.
     """
 
-    def __init__(self, env_fns: list[Callable]):
+    def __init__(
+        self,
+        env_fns: list[Callable],
+        observation_space: spaces.Space | None = None,
+        action_space: spaces.Space | None = None,
+    ):
         self._env_fns = env_fns
         self._env: gym.vector.AsyncVectorEnv | None = None
         self.num_envs = len(env_fns)
-        # Instantiate one env to expose spaces (no GPU — _ensure_env is lazy).
-        tmp = env_fns[0]()
-        self.observation_space = tmp.observation_space
-        self.action_space = tmp.action_space
-        self.single_observation_space = tmp.observation_space
-        self.single_action_space = tmp.action_space
-        tmp.close()
+        if observation_space is not None and action_space is not None:
+            self.observation_space = observation_space
+            self.action_space = action_space
+        else:
+            tmp = env_fns[0]()
+            self.observation_space = tmp.observation_space
+            self.action_space = tmp.action_space
+            tmp.close()
+        self.single_observation_space = self.observation_space
+        self.single_action_space = self.action_space
 
     def _ensure(self):
         if self._env is None:
@@ -507,6 +515,11 @@ def create_libero_envs(
         if not selected:
             raise ValueError(f"No tasks selected for suite '{suite_name}' (available: {total}).")
 
+        # All tasks in a suite share identical observation/action spaces.
+        # Probe once and reuse to avoid creating a temp env per task.
+        cached_obs_space: spaces.Space | None = None
+        cached_act_space: spaces.Space | None = None
+
         for tid in selected:
             fns = _make_env_fns(
                 suite=suite,
@@ -521,7 +534,11 @@ def create_libero_envs(
                 camera_name_mapping=camera_name_mapping,
             )
             if is_async:
-                out[suite_name][tid] = _LazyAsyncVectorEnv(fns)
+                lazy = _LazyAsyncVectorEnv(fns, cached_obs_space, cached_act_space)
+                if cached_obs_space is None:
+                    cached_obs_space = lazy.observation_space
+                    cached_act_space = lazy.action_space
+                out[suite_name][tid] = lazy
             else:
                 out[suite_name][tid] = env_cls(fns)
             print(f"Built vec env | suite={suite_name} | task_id={tid} | n_envs={n_envs}")

src/lerobot/scripts/lerobot_eval.py

@@ -526,7 +526,7 @@ def eval_main(cfg: EvalPipelineConfig):
 
     logging.info(colored("Output dir:", "yellow", attrs=["bold"]) + f" {cfg.output_dir}")
 
-    logging.info("Making environment.")
+    logging.info(f"Making environment (batch_size={cfg.eval.batch_size}, async={cfg.eval.use_async_envs}).")
     envs = make_env(
         cfg.env,
         n_envs=cfg.eval.batch_size,