FourierLayer() has instance methods that do not act on the instance.

[sascha-kirch]

FourierLayer() has methods like _matrix_product() or _change_to_channel_first() that do not require the instance of the class.
They have been simply put into a base class that is then inherited from.
So it is actually not really a base class but rather a collection of functions used in the child classes.

They could be turned into static methods.