Enable Debug Info in `CMakeLists.txt`

For people who are not super familiar with `pybind` or `cmake`, if you want to generate the debug information for the shared library, you will need to comment the line `pybind11_strip(ndarray_backend_cpu)` and then compile it with regular cmake debug flag. Otherwise, the debug info will get stripped and you could not use debugging tools like (gdb, valgrind, etc.). 