Overflow in bnf.py stage

[Abdurrahheem]

There is an overflow warning in while running python isp_pipeline.py with default raw image.

/openISP/model/bnf.py:33: RuntimeWarning: overflow encountered in ushort_scalars
  rdiff[i,j] = abs(img_pad[y+i,x+j] - img_pad[y+2, x+2])```

[Kuo-TingKai]

you can fix it by:

rdiff[i,j] = abs(img_pad[y+i,x+j].astype(int) - img_pad[y+2, x+2].astype(int))

I fixed this warning issue in this commit (not merged yet)