write out unneeded nest() call

[simonpcouch]

We call tidyr::nest() on grid information for each resample (or, depending on parallel_over, each resample vs model vs preprocessor combination). Taken together, this nesting takes up a good amount of time, especially so for fits to many resamples and quickly-running model fits.

In this case, writing that nest() call out saves us something like 8% of evaluation time. With main dev:

library(tidymodels)

bench::mark(
  res = fit_resamples(linear_reg(), mpg ~ ., bootstraps(mtcars, 100))
)
#> Warning: Some expressions had a GC in every iteration; so filtering is
#> disabled.
#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 res           3.14s    3.14s     0.319    46.6MB     8.60

With this PR:

#> # A tibble: 1 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 res           2.89s    2.89s     0.346    46.4MB     8.66

^{Created on 2023-03-15 with reprex v2.0.2}

[EmilHvitfeldt]

Looks good!

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