Add erf operators and unit tests

Context

While trying to export the ConvNeXt onnx model to cpp, we encountered several issues. The one of the issue was the missing operator erf.

Modified files

Added erf operator :

• erf.hpp added erf operator;
• erf_config.jinja, erf_forward.jinja template jinja file for erf;
• test_export.py, unit test for the erf operator;
• operator.py, register erf operator;

TODO

Added erf operator for ConvNeXt model

• NOT DONE
• DONE
• TO DO