You can get it from just printing below reshaped shape. As  torch.Size([128, 46656]) is flattened shape where
128: is no of batch size
46656 is tensor need to passed to the Linear in_features.
This only happens when you have input of shape [batch, channel, h,w] i.e [128,3,224,224] .  If you wanted to have different input shape , you can see reshape shape and use it accordingly.