[Feature Request] Counter Factual GCAM

[MoH-assan]

Hi,
It would be nice to support counterfactual grad-cams from section 3.3 in the original paper.
https://arxiv.org/pdf/1610.02391

I guess it is a matter of changing this

pytorch-grad-cam/pytorch_grad_cam/base_cam.py

Lines 141 to 142 in 1ff3f58

	cam = self.get_cam_image(input_tensor, target_layer, targets, layer_activations, layer_grads, eigen_smooth)
	cam = np.maximum(cam, 0)

to this

            cam = self.get_cam_image(input_tensor, target_layer, targets, layer_activations, layer_grads, eigen_smooth)
            if counter_factual: #bkr
                cam = cam * -1

and passing counter_factual through call all the way to compute_cam_per_layer

I can do a pull request, but I am not very familiar with the entire code base.

Thanks,