Fix Python build for activation_soft_max function

[An Long (aisk)]

There is a function overload for ActivationSoftmax, so the Python build will encounter this error:

    C:\Users\xxxxx\Source\DirectML\Python\src\module.cpp(557,12): error C2672: 'pybind11::module_::def': no matching overloaded function found [C:\Users\xxxxx\Source\DirectML\Python\build\temp.win-amd64-cpython-310\Release\pydirectml.vcxproj]
          C:\Users\xxxxx\Source\DirectML\Python\pybind11\include\pybind11\pybind11.h(1162,14):
          could be 'pybind11::module_ &pybind11::module_::def(const char *,Func &&,const Extra ...)'
              C:\Users\xxxxx\Source\DirectML\Python\src\module.cpp(557,12):
              'pybind11::module_ &pybind11::module_::def(const char *,Func &&,const Extra ...)': could not deduce template argument for 'Func'