Add PoolFormer
# What does this PR do?

This PR adds the PoolFormer model to the 🤗 repository.

I also opened an Issue for adding the model https://github.com/huggingface/transformers/issues/14584

# Who can review?
@NielsRogge 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for all your work on this, merging!