Move cfg.validate() earlier to fix NoneType error with --policy.path (#2782)

src/lerobot/scripts/lerobot_train.py

@@ -251,6 +251,8 @@ def train(cfg: TrainPipelineConfig, accelerator: Accelerator | None = None):
         cfg: A `TrainPipelineConfig` object containing all training configurations.
         accelerator: Optional Accelerator instance. If None, one will be created automatically.
     """
+    cfg.validate()
+
     # Create Accelerator if not provided
     # It will automatically detect if running in distributed mode or single-process mode
     # We set step_scheduler_with_optimizer=False to prevent accelerate from adjusting the lr_scheduler steps based on the num_processes
@@ -274,8 +276,6 @@ def train(cfg: TrainPipelineConfig, accelerator: Accelerator | None = None):
     # When using accelerate, only the main process should log to avoid duplicate outputs
     is_main_process = accelerator.is_main_process
 
-    cfg.validate()
-
     # Only log on main process
     if is_main_process:
         logging.info(pformat(cfg.to_dict()))