@yuxihu @apeforest 
I made a mistake in the example. The value of spatial_scale should be 1.0. 
So the statement 'mx.nd.ROIAlign(x, y, (2, 2), 0.7)' should be 
'mx.nd.ROIAlign(x, y, (2, 2), 1.0)'.
If you rerun the example, the output will be 
[[[[5.25, 6.75],
[14.25, 15.75]]]].

I think the output should be
[[[[3,5, 5.5],
[15.5, 17.5]]]]

Because y = mx.nd.array([[0,0,0,3,3]]) means the start coordinate is (0, 0) and 
the end coordinate is (3, 3).
The RoI is
[ 0., 1., 2., 3.],
[ 6., 7., 8., 9.],
[ 12., 13., 14., 15.],
[ 18., 19., 20., 21.]

Since pooled_size = (2, 2),  and both of the height and width of RoI are 4, 
which can be divisible by pooled_size. 
According to the description of ROIAlign, we can compute the each value of the 
output, for example,
3.5 = (0 + 1 + 6 + 7) / 4,  and so on.



[ Full content available at: 
https://github.com/apache/incubator-mxnet/issues/12403 ]
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