add `mobilebert` onnx configs
# What does this PR do?

This PR adds MobileBert OnnxConfig to make this model available for conversion. #16308 

## Who can review?

@lewtun @LysandreJik 

Anyone in the community is free to review the PR once the tests have passed.


_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @manandey thanks for the PR, it looks really clean. Did you try to convert one MobileBERT model with this config?

It could be nice to upload a converted MobileBERT model of your choice to the `ONNXConfig for all` if you have time.<|||||>Hi @lewtun, I tried to address the fixes you had suggested, and the tests are passing after running `RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -s -k "mobilebert" `.  :) 