Update LLM.generate output to include statistics

[plaguss]

Description

This PR updates the output from llm.generate to make it more feature rich.

Previously we only returned the generated text:

GenerateOutput = List[Union[str, None]]

Now it is updated to allow for statistics related to the generation:

LLMOutput = List[Union[str, None]]

class TokenCount(TypedDict):
    input_tokens: List[int]
    output_tokens: List[int]

LLMStatistics = Union[TokenCount, Dict[str, Any]]
"""Initially the LLMStatistics will contain the token count, but can have more variables.
They can be added once we have them defined for every LLM.
"""

class GenerateOutput(TypedDict):
    generations: LLMOutput
    statistics: LLMStatistics

This PR only includes input_tokens and output_tokens as statistics, but we can add as much as needed in the future.

This information is moved to distilabel_metadata in the following way, to avoid collisions between statistics of different steps:

{
    "generations": ["Hello Magpie"],
    f"statistics_{step_name}": {
        "input_tokens": [12],
        "output_tokens": [12],
    },
}

NOTE:
Most Task reuse the same Task.process method to process the generations, and nothing else has to be done, but for tasks like Magpie where the process method is overwritten, this has to be updated.

Closes #738

[github-actions]

Documentation for this PR has been built. You can view it at: https://distilabel.argilla.io/pr-1034/

[codspeed-hq]

CodSpeed Performance Report

Merging #1034 will not alter performance

_{Comparing llm-generate-upgrade (7c6e18f) with develop (e830e25)}

Summary

✅ 1 untouched benchmarks