Add scoring WER/BLEU in speech_to_text

[gegallego]

Before submitting

• [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x] Did you read the contributor guideline?
• [x] Did you make sure to update the docs?
• [ ] Did you write any new necessary tests?

What does this PR do?

It allows scoring validation outputs in speech_to_text with WER, BLEU or any other scoring metric.

[gegallego]

Sorry for the mess with closing/opening the PR, I just wanted to change the branch name in my fork 🙏