sft learn to generate eos token

[digitalSquirrel1]

Avoid 'eos_token' from being labeled as -100 and stopping contribute to loss.
one-line changing in sft.py.
correlated issue: #492

[digitalSquirrel1]

transformers.data.data_collator.DataCollatorForLanguageModeling:

[qgallouedec]

Thanks!

[javirandor]

@digitalSquirrel1 This works only when starting SFT from Qwen/Qwen2.5-Math-7B-Instruct. Other models have the same EOS and PAD token e.g. Qwen/Qwen2.5-Math-7B (see here) so this will not work.