small fixes

src/lerobot/scripts/lerobot_train.py

@@ -294,7 +294,7 @@ def train(cfg: TrainPipelineConfig, accelerator: Callable | None = None):
         sampler=sampler,
         pin_memory=device.type == "cuda",
         drop_last=False,
-        prefetch_factor=2,
+        prefetch_factor=2 if cfg.num_workers > 0 else None,
     )
     if accelerator:
         policy, optimizer, dataloader, lr_scheduler = accelerator.prepare(
@@ -369,6 +369,8 @@ def train(cfg: TrainPipelineConfig, accelerator: Callable | None = None):
                 policy=policy if not accelerator else accelerator.unwrap_model(policy),
                 optimizer=optimizer,
                 scheduler=lr_scheduler,
+                preprocessor=preprocessor,
+                postprocessor=postprocessor,
             )
             update_last_checkpoint(checkpoint_dir)
             if wandb_logger: