fix: RuntimeError: result type Float can't be cast to the desired output type long int

[ItzDerock]

This PR re-implements the patch done in ultralytics/yolov5 pr 8067 to fix this error when using torch 1.12

[ItzDerock]

fixes #270

[RainYQ]

for torch == 1.13.0

just fix it

utils/loss.py

-> 130
gain = torch.ones(6, device=targets.device, dtype=torch.long) # normalized to gridspace gain
-> 138
gain[2:] = torch.tensor(p[i].shape, dtype=torch.long)[[3, 2, 3, 2]] # xyxy gain
-> 144
at = torch.arange(na, device=targets.device).view(na, 1).repeat(1, nt) # anchor tensor, same as .repeat_interleave(nt)