chore(processor): Add default names for preprocessor and postprocessor in constants

- Introduced `PREPROCESSOR_DEFAULT_NAME` and `POSTPROCESSOR_DEFAULT_NAME` constants for consistent naming across various processor implementations.
- Updated processor creation in multiple policy files to utilize these constants, enhancing code readability and maintainability.
- Modified the training script to load and save the preprocessor and postprocessor using the new constants.

src/lerobot/utils/train_utils.py

@@ -32,6 +32,7 @@ from lerobot.datasets.utils import load_json, write_json
 from lerobot.optim.optimizers import load_optimizer_state, save_optimizer_state
 from lerobot.optim.schedulers import load_scheduler_state, save_scheduler_state
 from lerobot.policies.pretrained import PreTrainedPolicy
+from lerobot.processor.pipeline import RobotProcessor
 from lerobot.utils.random_utils import load_rng_state, save_rng_state
 
 
@@ -74,7 +75,8 @@ def save_checkpoint(
     policy: PreTrainedPolicy,
     optimizer: Optimizer,
     scheduler: LRScheduler | None = None,
-    preprocessor=None,
+    preprocessor: RobotProcessor | None = None,
+    postprocessor: RobotProcessor | None = None,
 ) -> None:
     """This function creates the following directory structure:
 
@@ -105,6 +107,8 @@ def save_checkpoint(
     cfg.save_pretrained(pretrained_dir)
     if preprocessor is not None:
         preprocessor.save_pretrained(pretrained_dir)
+    if postprocessor is not None:
+        postprocessor.save_pretrained(pretrained_dir)
     save_training_state(checkpoint_dir, step, optimizer, scheduler)