Create new validation_time_split() function

[juliasilge]

No description provided.

[juliasilge]

cc @mine-cetinkaya-rundel

[juliasilge]

Closes #251

library(rsample)
data(drinks, package = "modeldata")
rs <- validation_time_split(drinks)
rs
#> # Validation Set Split (0.75/0.25)  
#> # A tibble: 1 × 2
#>   splits           id        
#>   <list>           <chr>     
#> 1 <split [231/78]> validation
testing(rs$splits[[1]])
#> # A tibble: 78 × 2
#>    date       S4248SM144NCEN
#>    <date>              <dbl>
#>  1 2011-04-01           9281
#>  2 2011-05-01           9967
#>  3 2011-06-01          11344
#>  4 2011-07-01           9106
#>  5 2011-08-01          10469
#>  6 2011-09-01          10085
#>  7 2011-10-01           9612
#>  8 2011-11-01          10328
#>  9 2011-12-01          11483
#> 10 2012-01-01           7486
#> # … with 68 more rows

^{Created on 2021-09-20 by the reprex package (v2.0.1)}

This allows folks to do something like this:

library(rsample)
nested_cv(mtcars, outside = vfold_cv(v = 3), inside = validation_time_split(prop = 0.8))
#> # Nested resampling:
#> #  outer: 3-fold cross-validation
#> #  inner: Validation Set Split (0.8/0.2) 
#> # A tibble: 3 × 3
#>   splits          id    inner_resamples           
#>   <list>          <chr> <list>                    
#> 1 <split [21/11]> Fold1 <validation_split [1 × 2]>
#> 2 <split [21/11]> Fold2 <validation_split [1 × 2]>
#> 3 <split [22/10]> Fold3 <validation_split [1 × 2]>

^{Created on 2021-09-20 by the reprex package (v2.0.1)}

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