Add default structured output for GenerateSentencePair task

[plaguss]

Description

Adds a default structured output for GenerateSentencePair, it can be set from the Task and will prepare the LLM for it internally.

from distilabel.steps.tasks import GenerateSentencePair
from distilabel.llms import InferenceEndpointsLLM

generate_sentence_pair = GenerateSentencePair(
    triplet=True, # `False` to generate only positive
    action="query",
    llm=...,
#    llm=InferenceEndpointsLLM(
#        model_id="meta-llama/Meta-Llama-3.1-70B-Instruct",
#    ),
    use_default_structured_output=True,
    input_batch_size=10,
)

Depending of the LLM will use a provider for the structured outputs. For example with InferenceEndpointsLLM it will use outlines to generate a dict of the form: {"positive": ...} or {"positive": ..., "negative": ...} that will be easier to parse.

It also puts in place a mechanism to define a default structured output for every Task that may require it.

Closes #867

[github-actions]

Documentation for this PR has been built. You can view it at: https://distilabel.argilla.io/pr-868/

[gabrielmbmb]

LGTM!