Simplify the setup for AnchorGenerator in unittest

[zhiqwang]

Hi @fmassa , this is a follow-up PR of #2983 . Now there is one layer with grid_size (3x3), so the estimated number of anchors are 9. And the detailed values of the anchor is

torch.tensor([[-5., -5.,  5.,  5.],
              [ 0., -5., 10.,  5.],
              [ 5., -5., 15.,  5.],
              [-5.,  0.,  5., 10.],
              [ 0.,  0., 10., 10.],
              [ 5.,  0., 15., 10.],
              [-5.,  5.,  5., 15.],
              [ 0.,  5., 10., 15.],
              [ 5.,  5., 15., 15.]])

in this case. Does this meet the requirements?

[fmassa]

Much better, thanks a lot!