add case_weights argument to workflow_set

[simonpcouch]

library(workflowsets)
library(parsnip)

# setup
lr_spec <- linear_reg() %>% set_engine("lm")
knn_spec <- nearest_neighbor() %>%
   set_engine("kknn") %>%
   set_mode("regression")

cars <-
   mtcars %>%
   dplyr::mutate(
      wts = hardhat::importance_weights(runif(nrow(.))),
      non_wts = runif(nrow(.))
   )

# when done right:
workflow_set(
   list(reg = mpg ~ ., nonlin = mpg ~ wt + 1 / sqrt(disp)),
   list(lm = lr_spec),
   case_weights = wts
) %>%
   workflow_map(
      "fit_resamples",
      resamples = rsample::vfold_cv(cars, v = 5)
   )
#> # A workflow set/tibble: 2 × 4
#>   wflow_id  info             option    result   
#>   <chr>     <list>           <list>    <list>   
#> 1 reg_lm    <tibble [1 × 4]> <opts[1]> <rsmp[x]>
#> 2 nonlin_lm <tibble [1 × 4]> <opts[1]> <rsmp[x]>

# specify a column that is not case weights:
workflow_set(
   list(reg = mpg ~ ., nonlin = mpg ~ wt + 1 / sqrt(disp)),
   list(lm = lr_spec),
   case_weights = non_wts
) %>%
   workflow_map(
      "fit_resamples",
      resamples = rsample::vfold_cv(cars, v = 5)
   )
#> Error:
#> ! The supplied `case_weights` argument 'non_wts' is not a case weights column. See `?workflow_set` for more information.
#> Execution stopped; returning current results
#> # A workflow set/tibble: 2 × 4
#>   wflow_id  info             option    result    
#>   <chr>     <list>           <list>    <list>    
#> 1 reg_lm    <tibble [1 × 4]> <opts[1]> <list [0]>
#> 2 nonlin_lm <tibble [1 × 4]> <opts[1]> <list [0]>

# specify an engine that allows case weights:
workflow_set(
   list(reg = mpg ~ ., nonlin = mpg ~ wt + 1 / sqrt(disp)),
   list(lm = knn_spec),
   case_weights = wts
)
#> Warning: Case weights are not enabled by the underlying model implementation for the following engine(s): kknn.
#> 
#> The `case_weights` argument will be ignored for specifications using that engine.
#> # A workflow set/tibble: 2 × 4
#>   wflow_id  info             option    result    
#>   <chr>     <list>           <list>    <list>    
#> 1 reg_lm    <tibble [1 × 4]> <opts[0]> <list [0]>
#> 2 nonlin_lm <tibble [1 × 4]> <opts[0]> <list [0]>

^{Created on 2022-05-26 by the reprex package (v2.0.1)}

[simonpcouch]

cc @DavisVaughan—if you're game, let's review this PR in pairing on Monday. Would like to watch your process and also get a sense for whether there's a better way to subset into these objects in the case weights logic. I think some of the code points toward need for getters/indicators like has_case_weights or get_case_weights_name.

[DavisVaughan]

Partial review which we will continue on Tuesday

[DavisVaughan]

Looks good with a few comments

[simonpcouch]

Much appreciated, @DavisVaughan! Rest of changes resolved in 639c2ea.

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