Cleanup SKL LabelEncoder Converter Code

[ksaur]

We have the original draft of the SKL LabelEncoder 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 LabelEncoder here.

There is a test file here that needs to be cleaned up and passing (and also tests added for errors).

Notes:

• We had to treat strings a bit differently because of the way they are represented in tensors
  • We should add more expansive tests
• In the numeric label encoder, there is a tricky spot here related to running on GPU vs CPU.
  • There may be some bugfixes/improvements that are available now. Our CI/CD pipeline is not currently GPU-enabled, but it would be helpful to be able to run this on a GPU-enabled machine when developing.

[amueller]

Btw, many people that are using LabelEncoder should actually be using OneHotEncoder. LabelEncoder was meant for labels i.e. targets, not features, and only supports a single column, while OneHotEncoder supports multiple columns.

[interesaaat]

Good to see you here @amueller! :) Yes we also have OneHotEncoder, it is work in progress in PR #147.

[ksaur]

closed via #374