Reduce console spam when using the KerasMetricCallback
Right now, `KerasMetricCallback` calls `model.predict()` while iterating over the input dataset. This results in some unwanted console spam when using metrics that do not call `generate()` (because `predict()` always creates a progress bar). Replacing it with `predict_on_batch` removes the spam and also improves performance of the callback.
_The documentation is not available anymore as the PR was closed or merged._