Adding ONNX support for RobertaClassification

[danilojsl]

Description

Adding ONNX support for:

• RobertaForSequenceClassification
• RobertaForTokenClassification
• RobertaForQuestionAnswering

Motivation and Context

• Adding ONNX support for classification annotators for faster inference

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code improvements with no or little impact
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING page.
• I have added tests to cover my changes.
• All new and existing tests passed.