The input that you have passed to summary is a 4D tensor of size ([batch_size, 100, 64, 64]), so before passing that directly to the linear layer, you have to reshape to a 2D Tensor. just add:
 x = x.view(-1, 100)	//100 = input_channels to the Linear layer
before x = self.linear(x) in the forward() and you are good to go.