Add support for RemBERT in the ONNX export

[mlynatom]

Feature request

Add RemBERT to supported architectures for ONNX export.

Motivation

The support for RemBert was previously available in Transformers see here. However, now it seems that RemBERT is no longer supported.

Your contribution

I can help by testing implementation or providing the code if provided by some tutorial. I was not able to find documentation on how to do that.

[IlyasMoutawwakil]

Hi, I would love to review a PR, adding a model is usually something like https://github.com/huggingface/optimum/pull/1161/files
where we add a model config (in this case it will just inherit from bert text encoder config) and the appropriate hub models for testing.