Cleanup and import SKL Feature Pre-processing: OneHotEncoder

[ksaur]

We have the original draft of the OneHotEncoder code uploaded in this branch. All credit here goes to @scnakandala, the original author and brains behind this.

It contains an un-edited implementation of OneHotEncoder here.

There is a single test file here 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.

[TuanNguyen27]

hey @ksaur can i also work on this ? :)

[ksaur]

I did one small commit to the branch, just adding a flag that might be confusing and cause the code to not work; let me know if you run into issues or have questions!