Confusion about the forward function of the modified ResNet class

Hi, 

In the forward function of the ResNet class in `./mmdetection_variable/mmdet/models/backbones/resnet.py` why do you use this:

```python
nips_img.data = new_imgs
nips_bpp.data = bpg_bpps
```

This breaks the gradient backpropagation path and gradients coming from later layers will not propagate back through the image_compressor/pre_processor.
