Add Onnx Config for PoolFormer
# What does this PR do?

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Fixes # (https://github.com/huggingface/transformers/issues/16308)


Add changes to make PoolFormer models available for Onnx conversion.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ChainYo



I have ran 
```RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -k "poolformer"```

![tt](https://user-images.githubusercontent.com/51019420/198833650-39a4aa51-fb61-41fa-828d-d948e5942ad7.jpg)
<|||||>Conversion output

![tt2](https://user-images.githubusercontent.com/51019420/198833670-9728dfdc-4bdd-461e-958e-2350bd6923a5.jpg)
<|||||>@ChainYo @lewtun Any suggestions here?

Thanks and Regards.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19962). All of your documentation changes will be reflected on that endpoint.<|||||>Any updates on this PR?<|||||>@ChainYo Any updates here?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Hey @lewtun and @michaelbenayoun, we should merge this before the Optimum change. 

Could you help on this?<|||||>Hi @ChainYo,
The change is already here, I think the PR can be merged once the conflicts are resolved.
Also @BakingBrains could you add it to Optimum as well? It should not require much effort. If not, I can make it myself.<|||||>@michaelbenayoun Sure, I will add it to Optimum. 

Thank you<|||||>I tried to resolve the conflicts, but I think I messed up<|||||>I just reopened a new pull request for the same with resolved conflicts, can you please check @michaelbenayoun 
https://github.com/huggingface/transformers/pull/20868

Thank you