[R-package] respect aliases for objective and metric and lgb.train() and lgb.cv()

[jameslamb]

Similar to #4903, this PR proposes changes to ensure that parameter aliases in lgb.train() and lgb.cv() are handled correctly in the R package.

Currently, internal helper functions access params$objective and params$metric directly, without considering aliases.

LightGBM/R-package/R/utils.R

Line 168 in ce486e5

if (is.character(params$objective)) {

LightGBM/R-package/R/utils.R

Line 194 in ce486e5

if (is.null(params$metric)) {

As a result, it's not possible today to use any of the aliases for objective with lgb.train() and lgb.cv().

library(lightgbm)
data("agaricus.train", package = "lightgbm")
train <- agaricus.train
dtrain <- lgb.Dataset(
    data = train$data
    , label = train$label
)
bst <- lgb.train(
    data = dtrain
    , params = list(
        num_leaves = 5L
        , application = "binary"
        , num_iterations = 5L
    )
)

Error in lgb.check.obj(params = params, obj = obj) :
lgb.check.obj: objective should be a character or a function

This PR proposes the following changes to address that:

• populate params$objective and params$metric using internal helper function lgb.check.wrapper_param(), which handles reconciling parameters passed by main parameter name and aliases
• move code for handling the possibility of custom objective and metrics (which are R function objects, not strings) down so it happens after aliases have been handled in params

[StrikerRUS]

Could you please resolve a conflict with master?

[StrikerRUS]

LGTM, thank you!

[StrikerRUS]

I like this assertion! Can we add the same assertion for objective into test_that("lgb.train() respects parameter aliases for objective", test?

[jameslamb]

ah yep, definitely! good point that params should be checked for that test too.

[jameslamb]

added in f90b2d6

[github-actions]

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