W&B: track batch size after autobatch (#6039)

* track batch size after autobatch

* remove redundant import

* Update __init__.py

* Update __init__.py

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

utils/callbacks.py

@@ -32,7 +32,7 @@ class Callbacks:
             'on_fit_epoch_end': [],  # fit = train + val
             'on_model_save': [],
             'on_train_end': [],
-
+            'on_params_update': [],
             'teardown': [],
         }