Thank you Napam, that was insightful!
So for each input channel, we take the average over height and width and then also average over batch, so that we get a 3-element vector that basically holds the average red, the average green, and the average blue activation.
So, sticking to the documentation, the x can be seen as a 3-channel (let’s stick to RGB) image, the E[ ] and Var[ ] are taken with respect to height, width, and batch, and \gamma and \beta are applied element-wise to the 3 channels?