Why should the deformable group be set as "n_anchors" ?

[RubyLiao]

The channels of input and output of DCN are the same. Why should we add an offset of 2 * 9 * n_anchors and set the deformable groups of DCN as n_anchors?
中文：为什么要把不改变通道数的DCN设为n_anchors个group呢？没看懂其中意义。
请教~