@liammmm I too am recently interested in heatmaps. It seems that the cnn_learner includes a plot_top_losses() for which you can set heatmap = true. See https://docs.fast.ai/vision.learner.html#_cl_int_plot_top_losses
Here is a tutorial that talks about this, although it doesn’t really get to the heatmaps until the very end: https://heartbeat.fritz.ai/heatmaps-and-convolutional-neural-networks-using-fast-ai-16d5b7d02a86
And here’s an earlier thread about plot_top_losses: Heatmaps in plot_top_losses()
Apart from the plot_top_losses, one can define a hook into the model and use that to get information usable for a heatmap. Here are some notes taken from an earlier iteration of Fastai where the hooks are used for heatmaps: https://medium.com/@lankinen/fast-ai-lesson-6-notes-part-1-v3-646edf916c04
Probably there are even better, more current responses to this that others can supply, but this is what I know right now.