scorecam update: nan fix for batch size less than 4

[sgsangodkar]

NAN issue with batch_size <=4 for scorecam.

Steps to reproduce the issue are as follows:
In cam.py,
#model = models.resnet50(pretrained=True)
model = models.vgg16(pretrained=True)

#target_layers = [model.layer4[-1]]
target_layers = [model.features[-1]]

#cam.batch_size = 32
cam.batch_size = 4

The output is as shown here:
https://github.com/sgsangodkar/pytorch-grad-cam/blob/scorecam_update/bsize4_before_scorecam_cam.jpg

Observation:
Output is not proper since there are 'nan's in the cam weights due to min max normalisation in scorecam.py file.

Possible Fix is as follows:
#maxs, mins = maxs[:, :, None, None], mins[:, :, None, None]
#upsampled = (upsampled - mins) / (maxs - mins)

        mask = maxs!=mins
        maxs, mins = maxs[:, :, None, None], mins[:, :, None, None]
        upsampled[mask,:] = (upsampled[mask,:] - mins[mask,:]) / (maxs[mask] - mins[mask])

Output after the fix is as shown here:
https://github.com/sgsangodkar/pytorch-grad-cam/blob/scorecam_update/bsize4_after_scorecam_cam.jpg

Regards

Sagar