Fix to KerasMetricCallback when the model returns unstructured output
The `KerasMetricCallback` was only tested on `transformers` models, which usually return dict-like `ModelOutput`. As a result, I missed a bug when the model is a more classical Keras model that just returns a naked array. Thanks to @leadbetterben for pointing out the issue.

Fixes #21674 .
_The documentation is not available anymore as the PR was closed or merged._<|||||>I don't even know if we're testing this callback at all, because it's not really a core piece of `transformers`. I'll merge this for now and then think about where we could add some tests in a future PR!