text: Title: Error: Convert Pytorch model to IR model 
  Body: I trained the model with pytorch and used torch.save() to save the whole model including weights and network but when I run "mmtoir -f pytorch -d mn --inputShape 1,28,28 -n xxx.pth",  I got the error 
  
  > "AttributeError: Can't get attribute 'Net' on <module '__main__' from '/home/xxxx/anaconda3/bin/mmtoir'>" 
  
  > 
  It seems mmtoir cannot find my network definition?