fix typos in idefics.md
Author: dribnet
Pull request: yes

Two typos found in reviewing this documentation.

1) max_new_tokens=***6*** as ***4*** is not sufficient to generate "Vegetables" as indicated - you will get only "Veget". (incidentally - some mention of how to select this value might be useful as it seems to change in each example)

2) inputs = processor(prompts, return_tensors="pt"))***.to(device)*** as inputs need to be on the same device (as they are in all other examples on the page)

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## Before submitting
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Models:

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