Support custom results/details path

[albertvillanova]

Support custom results/details path.

This PR adds the optional output_path parameter to EvaluationTracker.save_results and EvaluationTracker.save_details:

• If None, same behavior as before.
• If passed, use this as the results output path.

Also, it makes optional the EvaluationTracker.output_dir attribute:

• If set, use it as the common dir for output results/details (as before).
• If None, the output results and details can be fully customized and no longer need of a common dir for both.

TODO:

• Docstrings

[HuggingFaceDocBuilderDev]

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[NathanHB]

so it will be overiding output_dir, this isn't very clear. Users should be able to set the output_path in a coherent way with results and details.

[albertvillanova]

I agree. The output_dir attribute is ignored if the user passes the output_path parameter.

One could make output_dir optional:

• If passed, current behavior.
• If None, then needs an output_path for all save_ methods.

[albertvillanova]

Or simpler: one could use output_dir as the base dir for the passed output_path (considered this as relative to output_dir.

[albertvillanova]

Also note that the previous approach forces the user to save results and details under a common output_dir. What about if this is not the case?

[albertvillanova]

I have made output_dir optional.