DistillerModuleList conversion: Handle models w. duplicate modules (#338)

* By duplicate modules we mean:
  self.relu1 = nn.Relu()
  self.relu2 = self.relu1
* The issue:
  The second module ('relu2') will not be returned by
  torch.nn.Module.named_modules/children()
* When converting to DistillerModuleList, in order to maintain the
  original order of modules and in order to have a correct mapping
  of names before/after the conversion - we need to take the duplicates
  into account
* Implemented an internal version of named_modules/children that includes
  duplicates
* Added test case for this + refactored the module list conversion tests