Update the state of first argument to have learned or fit from second argument, and compute a cross validated example of such a transform.
fit_prepare(vps, dframe, ..., weights = NULL, parallelCluster = NULL)
vtreat pipe step, object specifying fit.
data.frame, data to fit from.
not used, forces later arguments to bind by name.
optional, per-dframe data weights.
optional, parallel cluster to run on.
@return named list containing: treatments and cross_frame
Note: input vps is not altered, fit is in returned list.