text: Title: Cleanup and import SKL Feature Pre-processing: OneHotEncoder
  Body: We have the original draft of the OneHotEncoder code uploaded in this [branch](https://github.com/microsoft/hummingbird/tree/cleanup/one-hot-enc). All credit here goes to @scnakandala, the original author and brains behind this.
  
  It contains an un-edited implementation of OneHotEncoder [here](https://github.com/microsoft/hummingbird/blob/cleanup/one-hot-enc/hummingbird/ml/operator_converters/one_hot_encoder.py).
  
  There is a single test file [here](https://github.com/microsoft/hummingbird/blob/cleanup/one-hot-enc/tests/test_sklearn_one_hot_encoder_converter.py) that needs to be cleaned up and passing.
  
  Notes:
  * We had to treat strings a bit differently because of the way they are represented in tensors
    *  We should add more robust tests here
  * the `retain_order`  flag in `OneHotEncoder` is not yet implemented. We can wait on that for later unless someone gets inspired.