Update callbacks.py with __init__() (#5979)

Add __init__() function.

utils/callbacks.py

@@ -9,8 +9,9 @@ class Callbacks:
     Handles all registered callbacks for YOLOv5 Hooks
     """
 
+    def __init__(self):
         # Define the available callbacks
-    _callbacks = {
+        self._callbacks = {
             'on_pretrain_routine_start': [],
             'on_pretrain_routine_end': [],