Change `batch_size` descriptions to proper ones (#13422)
Change `batch_size` descriptions to proper ones (#13422)

* Change `batch_size` descriptions to proper ones

Since there're no gradients updated during `evaulate` and `predict` processes, changed their `batch_size` docstrings from `"Number of samples per gradient update"` to `"Number of samples per evaluation step"` and `"Number of samples to be predicted at once"`. (The sentence in fit remains unchanged.)

I hope this fix would change related auto-generated documents as well.

* Correct `callbacks` description docstrings

Corrected `callbacks` description docstrings in `evaluate_generator` and `predict_generator`: "List of callbacks to apply during training" -> "- during evaluation", "- during prediction".

keras-team/keras,keras-team/keras