fix AdamW import in Training loop notebook

[Letssharearow]

AdamW doesn't exist in the transformer library

instead you can import AdamW from torch.optim

This is alreayd done in the course repository:
https://github.com/huggingface/course/blob/main/chapters/en/chapter3/4.mdx

And as other commits also directly change the files in here, I am not sure if this comment is still relevant:
course: Open a PR directly on the course repo (https://github.com/huggingface/course)

@sgugger

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[Letssharearow]

realizing there were some hidden changes in the jupyter notebook, this is the relevant change

[Letssharearow]

And here, too. It's 2 import changes from 2 cells

[pcuenca]

Hello @Letssharearow, thank you for your contribution! Would you mind submitting just the relevant changes, rather than replacing the notebook? I can't even see the diff in github because it's too large:

For reference, here's how a related PR did it: https://github.com/huggingface/notebooks/pull/606/files

[Letssharearow]

@pcuenca
Thank you, I realised it but only wanted to do the effort if Im actually in the right place
I figured it out by eventually now. May I ask, do you know why it's not synced with the course repo?

[pcuenca]

Thank you!

[pcuenca]

why it's not synced with the course repo?

Not sure, to be honest. I don't think there's an automatic process to sync them, these are just code-only notebooks without much of the prose in the course. cc @burtenshaw as he may know more details.