Fix Sylvain's nits on the original KerasMetricCallback PR (#18300)
* Fix Sylvain's nits on the original PR
* Update src/transformers/keras_callbacks.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* Re-add "optional" to docstring
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>