Updated Object detection task notebooks

[sergiopaniego]

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What does this PR do?

Fixes #514.

I've updated the notebooks associated with the Object Detection Task. I've ported the content of the documentation in https://huggingface.co/docs/transformers/tasks/object_detection which works correctly and is updated

Who can review?

@merveenoyan and @stevhliu

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[stevhliu]

Hi, the notebooks are automatically generated from the .md files here. So to update the notebooks, you can just update the docs first and then the doc builder converts them to notebooks 😄