Fix callback imports

src/transformers/keras_callbacks.py

@@ -17,7 +17,7 @@
 logger = logging.getLogger(__name__)
 
 
-class KerasMetricCallback(keras.Callback):
+class KerasMetricCallback(keras.callbacks.Callback):
     """
     Callback to compute metrics at the end of every epoch. Unlike normal Keras metrics, these do not need to be
     compilable by TF. It is particularly useful for common NLP metrics like BLEU and ROUGE that require string
@@ -265,7 +265,7 @@ def generation_function(inputs, attention_mask):
         logs.update(metric_output)
 
 
-class PushToHubCallback(keras.Callback):
+class PushToHubCallback(keras.callbacks.Callback):
     """
     Callback that will save and push the model to the Hub regularly. By default, it pushes once per epoch, but this can
     be changed with the `save_strategy` argument. Pushed models can be accessed like any other model on the hub, such