Solved by ZimoNitrome in post #2 
              
              
                It means any dimensions preceeding the ones listed. In this context it means that Grayscale() can process tensors such as: 
A single image with shape [3, H, W] (no leading dimensions) 
A batch of images of shape [B, 3, H, W] (here B is a leading dimension) 
A number of batches of shape [N, B, 3, H, …