[SFT] Log mean token accuracy from Liger kernel

[kashif]

What does this PR do?

We log the mean token accuracy from Liger kernel if it is available

Requires linkedin/Liger-Kernel#910

to test do:

pip install git+https://github.com/kashif/Liger-Kernel.git@mean_token_accuracy
pip install git+https://github.com/huggingface/trl.git@iiger-mean-acc

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[HuggingFaceDocBuilderDev]

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[qgallouedec]

I prefer using an explicit condition, like this:

if self.args.use_liger_kernel and version("liger_kernel") >= "0.6.4":
    token_accuracy = self.accelerator.gather_for_metrics(outputs.token_accuracy).mean().item()
    self._metrics[mode]["mean_token_accuracy"].append(token_accuracy)

Having the explicit condition makes it much easier to spot when the check can be removed after we bump the minimum liger_kernel version. It’s clearer and more maintainable than hiding the logic elsewhere.

[qgallouedec]

the question is then, which version will include this feature