I’m sure boxes is a tensor, its value is as following:
tensor([[230.1149, 237.2053, 456.3304, 232.7983, 456.9069, 262.5309, 230.6913,
         266.9381, 202.8954],
        [331.7273, 324.1699, 679.4318, 317.2317, 680.0470, 348.1467, 332.3426,
         355.0850,  88.9529],
        [231.7705, 204.8157, 500.3940, 200.7935, 500.8643, 232.1562, 232.2409,
         236.1783,  60.0000],
        [231.7874, 157.3486, 441.9472, 153.3222, 442.4890, 181.6230, 232.3291,
         185.6494,  47.9981],
        [230.4137,  63.0557, 326.3553,  59.4965, 327.4994,  90.2845, 231.5578,
          93.8438,  40.9682],
        [152.8475, 326.7796, 311.2224, 324.2658, 311.6898, 353.6737, 153.3150,
         356.1875,  28.9999],
        [154.1073, 113.9810, 260.5620, 112.2871, 260.9745, 138.2966, 154.5198,
         139.9904,  27.9924],
        [293.3543, 111.8462, 390.4526, 108.3323, 391.4826, 136.8042, 294.3844,
         140.3180,  13.9621],
        [152.4046, 159.5459, 219.9668, 157.7393, 220.6146, 181.9012, 153.0524,
         183.7079,  12.9934],
        [153.1970, 206.8427, 218.1942, 205.0301, 218.8990, 230.2958, 153.9019,
         232.1084,   7.0000],
        [153.0565,  68.3798, 217.7016,  67.7295, 217.9489,  92.0420, 153.3038,
          92.6922,   6.9623]], grad_fn=<IndexBackward>)

Its type and shape is:
torch.FloatTensor torch.Size([11, 9])