[FEA] Multi-GPU preprocessing.LabelEncoder

[dawilliams-nvidia]

Is your feature request related to a problem? Please describe.
Many of the preprocessing techniques are listed as Single-GPU only: https://docs.rapids.ai/api/cuml/stable/api.html#preprocessing-metrics-and-utilities

Some do have dask versions, however in particular LabelEncoder (https://docs.rapids.ai/api/cuml/stable/api.html#feature-and-label-encoding-single-gpu) does not seem to have an analogous Multi-GPU option (going from 'a','b','a','c' -> 0,1,0,2). The closest I've found is the LabelBinarizer which provides one hot encodings.

Describe the solution you'd like
Dask implementation of LabelEncoder

Additional context
NVIDIA Solutions Architect, filing on behalf of customer