Hardcoded pyramid_levels causing runtime error with stride customization

[hanoglu]

There is no option in the training code to change the stride variable. Apparently, stride depends on pyramid_levels value. But this value is hardcoded in the line self.anchor_points = AnchorPoints(pyramid_levels=[3,], row=row, line=line) and in case it is changed, it gives this runtime error; RuntimeError: The size of tensor a (1024) must match the size of tensor b (256) at non-singleton dimension at output_coord = regression + anchor_points