Update request on 'A full training loop' section of Chapter 3 of LLM course

[adover134]

1. Added process bar to evaluation loop.
2. At the end of 'Model Evaluation' of 'Next Steps and Best Practices' section, added a consideration to encourage about 'using accelerator with evaluation loop'.

[adover134]

It can be OK to skip notifying about 'accelerator with evaluation loop' if everyone reading and studying on the course is perfect student; but I think it is easy to overlook. Also, it will be much cool if we give progress bar also on the evaluation loop.

[HuggingFaceDocBuilderDev]

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