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/policies/diffusion/processor_diffusion.py

@@ -16,6 +16,7 @@
 # limitations under the License.
 import torch
 
+from lerobot.constants import POSTPROCESSOR_DEFAULT_NAME, PREPROCESSOR_DEFAULT_NAME
 from lerobot.policies.diffusion.configuration_diffusion import DiffusionConfig
 from lerobot.processor import (
     DeviceProcessor,
@@ -46,6 +47,6 @@ def make_diffusion_processor(
             features=config.output_features, norm_map=config.normalization_mapping, stats=dataset_stats
         ),
     ]
-    return RobotProcessor(steps=input_steps, name="robot_preprocessor"), RobotProcessor(
-        steps=output_steps, name="robot_postprocessor"
+    return RobotProcessor(steps=input_steps, name=PREPROCESSOR_DEFAULT_NAME), RobotProcessor(
+        steps=output_steps, name=POSTPROCESSOR_DEFAULT_NAME
     )