ONNX FARMReader model conversion is broken

[vblagoje]

Describe the bug
During a recent language and tokenization refactoring we broke FarmReader ONNX conversion support.

Error message
A simple ONNX conversion of a QA model fails; we don't have a test case for it.

onnx_model_path = Path("roberta-onnx-model")
FARMReader.convert_to_onnx(model_name="deepset/roberta-base-squad2", output_path=onnx_model_path)

Expected behavior
Model is converted to ONNX format, written to a local directory and subsequently used to create an ONNX-based reader.

Additional context
None

To Reproduce
Invoke the above-mentioned code sample in the Python interpreter

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System:

• OS:
• GPU/CPU:
• Haystack version (commit or version number):
• DocumentStore:
• Reader:
• Retriever: