text: Title: Improper converter check
  Body: Can someone please check this [converter check](https://github.com/microsoft/hummingbird/blob/2705c42585ac1d3ee69e65a93e5f59d7d42f44d5/hummingbird/ml/_topology.py#L205) in topology.py?
  Shouldn't it be a `if converter is None:` ?
  
  Or am I just missing something in the flow?
  