Add stop_training=False flag to callbacks (#6365)

* New flag 'stop_training' in util.callbacks.Callbacks class to prematurely stop training from callback handler

* Removed most of the new  checks, leaving only the one after calling 'on_train_batch_end'

* Cleanup

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/callbacks.py

@@ -35,6 +35,7 @@ class Callbacks:
             'on_params_update': [],
             'teardown': [],
         }
+        self.stop_training = False  # set True to interrupt training
 
     def register_action(self, hook, name='', callback=None):
         """