convert median to pnnx

[number8868]

mark

[tencent-adm]

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[nihui]

CMake Error at src/CMakeLists.txt:602 (add_executable):
  Cannot find source file:

    pass_level2/torch_median.cpp

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx


CMake Error at src/CMakeLists.txt:602 (add_executable):
  No SOURCES given to target: pnnx


CMake Generate step failed.  Build files cannot be regenerated correctly.