expose n_jobs for rlearner

[ZiJiaW]

Proposed changes

The Rlearner interface uses cross_val_predict to predict mu but did not expose n_jobs params. The default value of n_jobs uses all processors, which may consumes too much memory (to OOM) especially for large dataset. The cross_val_predict method uses joblib for parallelization, the memory will grow with the number of workers. It's more reasonable to expose this param and let users decide.

Types of changes

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Checklist

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Further comments

Nope

[jeongyoonlee]

LGTM. Thanks for your contribution.