Add relative action processor steps

Add disabled relative_actions_processor and absolute_actions_processor steps to the processor pipeline so lerobot-train can enable use_relative_actions via processor overrides.

policy_postprocessor.json

@@ -13,6 +13,12 @@
         }
       }
     },
+    {
+      "registry_name": "absolute_actions_processor",
+      "config": {
+        "enabled": false
+      }
+    },
     {
       "registry_name": "device_processor",
       "config": {

policy_preprocessor.json

@@ -11,6 +11,16 @@
       "registry_name": "to_batch_processor",
       "config": {}
     },
+    {
+      "registry_name": "relative_actions_processor",
+      "config": {
+        "enabled": false,
+        "exclude_joints": [
+          "gripper"
+        ],
+        "action_names": null
+      }
+    },
     {
       "registry_name": "normalizer_processor",
       "config": {