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>

train.py

@@ -138,6 +138,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
     # Batch size
     if RANK == -1 and batch_size == -1:  # single-GPU only, estimate best batch size
         batch_size = check_train_batch_size(model, imgsz)
+        loggers.on_params_update({"batch_size": batch_size})
 
     # Optimizer
     nbs = 64  # nominal batch size