🪂 Don't gather logits in SFT to avoid hanging

[qgallouedec]

What does this PR do?

Fixes #2879

I'm not sure why it does always hang, but it seems that at some point in the training, always the same, it can hang while trying to gather the logits. The fix consist in gathering only the number of correct tokens and the total number of tokens.

The only way was to copy the content of compute_token_accuracy in the SFTTrainer, which I think should be ok as it is only used once in the codebase.

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[coding-famer]

I guess the reason is the sizes of gathered logits(seq length) do not match. So gathering only the number of correct tokens and the total number of tokens will be a good fix.