added 2 ops: DownsampleFactorMax and DownsampleFactorMaxGrad on the new gpu back end

Don't use `blas_api.h` here.  You aren't calling any blas function, nor calling `import_pygpu__blas()`.  Also, there is no guarantee that this header will continue to accidentally include everything you need.

It looks like you only need `numpy_compat.h`.