Which layer should I use for visualizing by Grad-CAM?

Thank you for your work!
I am impressed with the integration of the visualization technique GradCAM!

I am currently applying GradCAM to Kinetics.
I am wondering which layer I should use for visualization.
When I read Grad-CAM paper, the author recommended that we generally use the last layer before the fully connected layer.

Could you tell me which layer we should use?
Also, could you provide me which specific layer name we should use?

https://github.com/facebookresearch/SlowFast/blob/fd41618191d3c21c1ca21a61369ce9917646cf9c/slowfast/config/defaults.py#L655