[data_loaders] Make DataLoader pickable

This will allow to pass an instance of it in
a queue when using the multiprocessing module.

beat/backend/python/data_loaders.py

@@ -310,6 +310,17 @@ class DataLoader(object):
 
         return (result, indices[0], indices[1])
 
+    def __getstate__(self):
+        state = self.__dict__.copy()
+
+        #  reset the data cached as its content is not pickable
+        for infos in state["infos"].values():
+            infos["data"] = None
+            infos["start_index"] = -1
+            infos["end_index"] = -1
+
+        return state
+
 
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