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

Add __init__() function.

Author: glenn-jocher

utils/callbacks.py

@@ -9,31 +9,32 @@ class Callbacks:
     Handles all registered callbacks for YOLOv5 Hooks
     """
 
-    # Define the available callbacks
-    _callbacks = {
-        'on_pretrain_routine_start': [],
-        'on_pretrain_routine_end': [],
+    def __init__(self):
+        # Define the available callbacks
+        self._callbacks = {
+            'on_pretrain_routine_start': [],
+            'on_pretrain_routine_end': [],
 
-        'on_train_start': [],
-        'on_train_epoch_start': [],
-        'on_train_batch_start': [],
-        'optimizer_step': [],
-        'on_before_zero_grad': [],
-        'on_train_batch_end': [],
-        'on_train_epoch_end': [],
+            'on_train_start': [],
+            'on_train_epoch_start': [],
+            'on_train_batch_start': [],
+            'optimizer_step': [],
+            'on_before_zero_grad': [],
+            'on_train_batch_end': [],
+            'on_train_epoch_end': [],
 
-        'on_val_start': [],
-        'on_val_batch_start': [],
-        'on_val_image_end': [],
-        'on_val_batch_end': [],
-        'on_val_end': [],
+            'on_val_start': [],
+            'on_val_batch_start': [],
+            'on_val_image_end': [],
+            'on_val_batch_end': [],
+            'on_val_end': [],
 
-        'on_fit_epoch_end': [],  # fit = train + val
-        'on_model_save': [],
-        'on_train_end': [],
+            'on_fit_epoch_end': [],  # fit = train + val
+            'on_model_save': [],
+            'on_train_end': [],
 
-        'teardown': [],
-    }
+            'teardown': [],
+        }
 
     def register_action(self, hook, name='', callback=None):
         """