Keras callback for prompting metrics
# What does this PR do?

It's a Keras callback that will print metrics at the end of every epoch. 
It's tested on summarization example in the examples.


## Who can review?

@Rocketknight1 @LysandreJik 

Looks good so far! Here's what I'm thinking:

1. We should probably use the Callback's `self.model` instead of expecting a global `model` object - that might work in notebooks, but it's not great otherwise.
2. Looking at it, maybe you were right that we're asking the user to do too much work, and we should make predictions from the model in this callback too. It's definitely trickier, though, because Keras doesn't like concatenating variable-length predictions together. Maybe our code can predict on batches, break the batches into lists and then make big lists of predictions and labels?<|||||>Migrated to here: https://github.com/huggingface/transformers/pull/14867