Your code has no initializer __init__.  Try the following code:
import torch
import torch.nn as nn
from collections import OrderedDict

class Discriminator(nn.Module):
    def __init__(self):
        super(Discriminator, self).__init__()
        self.net = nn.Sequential(OrderedDict([
          ('conv1', nn.Conv2d(1,20,5)),
          ('relu1', nn.ReLU()),
          ('conv2', nn.Conv2d(20,64,5)),
          ('relu2', nn.ReLU())
        ]))

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

d = Discriminator()
print(d.net.conv1)