Add onnx export for efficientnet

[Thas-Tayapongsak]

What does this PR do?

I added an onnx config class for efficientnet models. I tested it according to https://huggingface.co/docs/optimum/exporters/onnx/usage_guides/contribute on tiny random models and it passed, but for larger one like google/efficientnet-b7 it did not pass the atol.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@fxmarty, @echarlaix, @JingyaHuang, @michaelbenayoun

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[IlyasMoutawwakil]

@Thas-Tayapongsak please resolve conflicts (rebase or merge main)

[Thas-Tayapongsak]

I'm not very familiar with git yet but I see a green checkmark, so I hope what I did is correct.

[IlyasMoutawwakil]

Doesn't look right 🫠 The code changes went crazy, you should do:

git ferch
git merge main
*resolve conflicts if any*
git push

[Thas-Tayapongsak]

😬 my bad. I'll try again

[Thas-Tayapongsak]

I did it. seems better than last time

[echarlaix]

LGTM, thanks for the addition @Thas-Tayapongsak !