Fix issue 1058

[reuvenperetz]

Pull Request Description:

Fix an issue that imports onnx in the exporter during init if torch was found:

• In fakely_quant_onnx_pytorch_exporter.py, import onnx-related modules only if onnx was found. Add "dummy" FakelyQuantONNXPyTorchExporter to raise an error if it was used without Onnx installed.
• Move DEFAULT_ONNX_OPSET_VERSION to pytorch_export_facade.py to use it only there since it's in the pytorch_export_facade api.

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).