Add an option to set gcFirst to FALSE in the internal calls to system.time in form_form and xy_xy

[fabrice-rossi]

I am not sure if this is a bug or if this should be handled as a feature. In fit_helpers, both form_form and xy_xy call system.time with the default value of TRUE for gcFirst. This triggers unnecessary garbage collection which slows down significantly e.g. tune_grid for simple models such as decision trees. For instance the following code:

library(tidymodels)
library(mlbench)
data(PimaIndiansDiabetes)

my_grid <- expand.grid(min_n=2:50)
cv_folds <- vfold_cv(PimaIndiansDiabetes, v = 5, strata="diabetes")
my_model <- decision_tree(cost_complexity=0, min_n=tune()) %>% 
    set_engine("rpart",xval=0) %>% set_mode("classification")

tune_results <-  my_model %>% tune_grid(diabetes~.,
                                        resamples=cv_folds,
                                        grid=my_grid,
                                        metrics=metric_set(accuracy))

runs in roughly 50 seconds on my computer with gcFirst=TRUE but takes only 20 seconds with gcFirst=FALSE.

I'm not sure whether precise timing reporting is needed. If this is not the case, maybe gcFirst=FALSE should always be used. If some flexibility is needed, it could be a user visible option. In any case, the default to gcFirst=TRUE is adding a significant overhead to simple model fitting.

[topepo]

We'll turn off the garbage collection. We'll also tie the computing of the execution time to the control option for verbosity so that the new default will be to not compute the execution time.

[github-actions]

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