Refactor vocabulary info in recipes

[cbalioglu]

This PR bundles three features:

• Removes vocab_info from SequenceModel and Seq2SeqModel and relies on the passed tokenizer to infer the vocabulary information.
• Per user feedback, fairseq2.recipes.common functions now expect as argument specific configuration sections instead of the general recipe configuration for better readability and mypy compatibility.
• Removes the _add_ directive for configuration handling and extends _set_ to handle both adding and setting configuration keys. This also makes the logic consistent with _set_ being the equivalent of dict[key] = value and _del_ being the equivalent of del dict[key].