Codeparrot cleanup

[lvwerra]

This PR fixes one bug and does some cosmetic changes:

Bug:

• An os import was missing.

Other changes:

• Remove checkpoint blocks
• Remove wandb plot (would require us to set the (messy and still active) wandb project public just to show a single plot
• Add note about large models
• Add note about large dataset loading
• Change changes the tokenizer loading name from codeparrot to transformersbook/codeparrot. While the former works if you trained and saved a tokenizer it fails if you didn't wheras the latter solution always works. So it is not a bug but I think it will reduce confusion.

Let me know what you think!

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[lvwerra]

Updated the notes and added the wandb installation. Let me know what you think @lewtun

[lewtun]

LGTM! Merging it now :)