[question] Support Grad-CAM in MMYOLO's yolov5

[hhaAndroid]

Hi. Thanks a lot for your open-source library. I currently have grad-free and grad-cam support in MMYOLO, but I'm not sure if the grad-cam implementation is correct. I found that the feature maps and corresponding gradients are quite different, which leads to ineffective weighting. I would like to ask your opinion on this issue. Looking forward to your reply！

We also welcome you to discuss this!