Add support for Ernie model

[BrightXiaoHan]

Solve #555
This submission enables optimum to support the Ernie family model and support onnxruntime export of related models. The onnx code related to this model has been verified.

https://github.com/huggingface/transformers/tree/main/src/transformers/models/ernie

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?

[fxmarty]

LGTM thank you, waiting for the tests to run

Two things:

• could you add Ernie to the list at https://huggingface.co/docs/optimum/main/en/exporters/onnx/overview (in the docs/ folder)
• could you add ernie to

  optimum/tests/onnxruntime/test_modeling.py

  Line 1979 in 23fe6f7

  class ORTModelForFeatureExtractionIntegrationTest(ORTModelTestMixin):

  & here?

[fxmarty]

@BrightXiaoHan Could you add ernie as well to

optimum/tests/exporters/exporters_utils.py

Line 82 in 7e08a82

"esm": "hf-internal-testing/tiny-random-EsmModel",

? Also can you use https://huggingface.co/hf-internal-testing/tiny-random-ErnieModel instead of https://huggingface.co/hf-internal-testing/tiny-random-ErnieMModel? They appear to be two different model types in transformers and the test currently fail due to this.