[bugfix] Make source build work for systems where the default is python2 (#3718)

* Make compatible for system where the default is python2

* Update cmakelists with find package

* Switch to find_program
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>

tensoradapter/pytorch/CMakeLists.txt

@@ -2,12 +2,12 @@ cmake_minimum_required(VERSION 3.5)
 project(tensoradapter_pytorch C CXX)
 
 # Find PyTorch cmake files and PyTorch versions with the python interpreter $PYTHON_INTERP
-# (or "python" if empty)
-file(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/find_cmake.py FIND_CMAKE_PY)
+# ("python3" or "python" if empty)
 if(NOT PYTHON_INTERP)
-  set(PYTHON_INTERP python)
+  find_program(PYTHON_INTERP NAMES python3 python)
 endif()
 message(STATUS "Using Python interpreter: ${PYTHON_INTERP}")
+file(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/find_cmake.py FIND_CMAKE_PY)
 execute_process(
   COMMAND ${PYTHON_INTERP} ${FIND_CMAKE_PY}
   OUTPUT_VARIABLE TORCH_PREFIX_VER