The first nn.Flatten() layer in self.MobileNet_ConvAdd_conv1 would flatten the incoming tensor, which will create a shape mismatch in the following nn.Conv2d.
nn.X2d layers expect an input activation of [batch_size, channels, height, width], while the nn.Linear layer expects an activation of [batch_size, in_features] (in the default setup).
Remove the first Flatten layer and make sure you really want to use nn.BatchNorm1d in the second nn.Sequential container.