Remove global average pooling out of "forward_head"

[abcsimple]

Great job on your work! I have a small suggestion regarding the code in inceptionnext.py. It would be more convenient if the line "x = x.mean((2, 3)) # global average pooling" was moved from the "self.forward_head" function to the end of the "self.forward_features" function. This way, we can directly command the line "x = self.forward_head(x)" when we have our own classification layer, without needing to keep the "MlpHead" class and command the lines after "x = x.mean((2, 3)) # global average pooling". The current setup is a little inconvenient.