[Examples] Check key exists in datasets first
Author: oToToT
Pull request: yes

# What does this PR do?

Correctly check the key exists before accessing it in some example scripts. I guess this is probably a mistake when writing example scripts.

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## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?  

No tests since I didn't see any tests related to examples. Maybe someone could point it out for me.

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Probably @bhadreshpsavani would like to review this according to the log from `git blame`.
Or @sgugger, @patil-suraj as this is about examples.

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Models:

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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
Hi @oToToT,
Thanks for the PR, The changes seem valid to me. It should be as per changes in this PR.
@sgugger what's your view on these changes?

Hi, @sgugger
I'm not familiar with how huggingface do with pull requests, did I miss something to make it be merged?
Or I just need to stay and wait it.

Thanks!

Thanks for the ping! I just forgot to click the merge button 🤦