The forward method of MnistAlexnet contains an error.
Currently you are trying to call super(MnistAlexnet,self).forward(x), i.e. the forward method of nn.Module, which is not implemented.
Instead just use:
class MnistAlexnet(nn.Module):

        def __init__(self,alexnet):
            super(MnistAlexnet,self).__init__()
            self.alexnet=alexnet

        def forward(self,x):
            return self.alexnet(x)

You don’t need to apply the softmax on your outputs as nn.CrossEntropyLoss expects logits and applied F.log_softmax and nn.NLLLoss internally.
Also, you shouldn’t call forward directly, as this might break hooks, so just call the module directly.
PS: I’ve formatted your code to be able to copy it directly for debugging.
You can add code by wrapping it in three backticks ```