[Docs] Fix typo in CustomTrainer compute_loss method and adjust loss reduction logic
Author: MilkClouds
Pull request: yes

# What does this PR do?

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Fixes error in loss reduction logic in docs. source's implementation is correct but docs was wrong, you may refer to following.

https://github.com/huggingface/transformers/blob/9bec2654ed5b4ac43e880dc7e3cb2c18aeae70a9/src/transformers/loss/loss_utils.py#L28-L42

https://github.com/huggingface/transformers/blob/a1ad9197c5756858e9014a0e01fe5fb1791efdf2/docs/source/en/trainer.md#L189-L201

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- [ ] Did you write any new necessary tests?


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Comments:
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