Demo BLEU in the spanish-english translation example

[mattdangerw]

Once we land the BLEU metric, let's add BLEU to the mix of metrics we are showing off here: https://keras.io/examples/nlp/neural_machine_translation_with_keras_nlp/#evaluating-our-model-quantitative-analysis

[mattdangerw]

@abheesht17 does this make sense to you? Would you like to add this as a minor improvement?

[abheesht17]

It does! Will add it :)