better detection of groups/tuning parameters

When using data frames generated from the tune_*() functions, we silently produce a single plot/analysis if the user doesn't correctly specify what they want.

We should detect this (when there is more than one config) and produce a meaningful error.

Also, the plot functions have a group argument and the estimation functions require group_by(). That's confusing.

Example:

library(tidymodels)
library(probably)
#> 
#> Attaching package: 'probably'
#> The following objects are masked from 'package:base':
#> 
#>     as.factor, as.ordered
library(bonsai)

tidymodels_prefer()
theme_set(theme_bw())
options(pillar.advice = FALSE, pillar.min_title_chars = Inf)

set.seed(1345)
cls_train <- sim_classification(1000)
cls_test  <- sim_classification( 500)
cls_calib <- sim_classification( 500)

set.seed(7378)
cls_rs <- vfold_cv(cls_train)

lgb_spec <- boost_tree() %>% set_mode("classification") %>% set_engine("lightgbm")

cls_metrics <- metric_set(brier_class, roc_auc)

set.seed(6929)
lgb_tune_res <-
  boost_tree(min_n = tune()) %>%
  set_mode("classification") %>%
  set_engine("lightgbm") %>%
  tune_grid(
    class ~ .,
    resamples = cls_rs,
    control = control_resamples(save_pred = TRUE),
    metrics = cls_metrics,
    grid = tibble(min_n = c(2, 50))
  )

df_pred_res  <- lgb_res %>% collect_predictions()
#> Error in collect_predictions(.): object 'lgb_res' not found
df_pred_tune_res  <- lgb_tune_res %>% collect_predictions()

df_new <- df_pred_res[1:5,]
#> Error in eval(expr, envir, enclos): object 'df_pred_res' not found
df_tune_new <- df_pred_tune_res %>% dplyr::slice(1:5, .by = .config)

# Plotting issues

# This produces 1 plot; should be two
df_pred_tune_res %>%
  cal_plot_windowed(truth = class, estimate = .pred_class_1,
                    window_size = 0.1, step_size = 0.025)

# Using `group` makes two plots
df_pred_tune_res %>%
  cal_plot_windowed(truth = class, estimate = .pred_class_1, group = .config,
                    window_size = 0.1, step_size = 0.025)

# Estimation issues

# Should have two groups
df_pred_tune_res %>%
  cal_estimate_logistic(truth = class)
#> 
#> ── Probability Calibration
#> Method: Logistic Spline
#> Type: Binary
#> Source class: Data Frame
#> Data points: 2,000
#> Truth variable: `class`
#> Estimate variables:
#> `.pred_class_1` ==> class_1
#> `.pred_class_2` ==> class_2

# Has two groups via a different "by" mechanism:
df_pred_tune_res %>%
  group_by(.config) %>% 
  cal_estimate_logistic(truth = class)
#> 
#> ── Probability Calibration
#> Method: Logistic Spline
#> Type: Binary
#> Source class: Data Frame
#> Data points: 2,000, split in 2 groups
#> Truth variable: `class`
#> Estimate variables:
#> `.pred_class_1` ==> class_1
#> `.pred_class_2` ==> class_2

^{Created on 2023-03-21 by the reprex package (v2.0.1)}