[R-package] avoid unnecessary computation of std deviations in lgb.cv()

[jameslamb]

lgb.cv() in the R package performs k-fold cross-validation, and can optionally compute the standard deviation of evaluation metrics across all folds. This option is controlled by the argument showsd (default TRUE).

Today, even when showsd = FALSE, an internal function called by lgb.cv() still calculates error standard deviations and then lgb.cv() just ignores them.

LightGBM/R-package/R/lgb.cv.R

Lines 388 to 390 in 9143003

	if (showsd) {
	env$eval_err_list <- merged_msg$eval_err_list
	}

This PR proposes a fix that avoids this unnecessary work, which should slightly speed up lgb.cv() for users who are not interested in the standard deviation of error estimates. It also introduces a test confirming that the output format of lgb.cv() changes based on the value of the showsd argument.

[StrikerRUS]

Nice catch! LGTM!

[github-actions]

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