register_backward_hook is deprecating in torch 1.8

register_backward_hook is used in class_activation_map and is deprecating, it should be updated.

repro:
```
docker run --gpus all --rm -ti --ipc=host --net=host  projectmonai/monai:latest
python -m tests.test_vis_gradcam
```
output:
```
Using a non-full backward hook when the forward contains multiple autograd Nodes is deprecated and will be removed in future versions. 
```
