[BUG] result indices in group_argmin was not initialized to -1 as comment says

[thirtiseven]

Describe the bug
In sorted-based group_argmin 's comment it says that the indices was initialized to ARGMIN_SENTINEL but it was not in the current code.

cudf/cpp/src/groupby/sort/group_argmin.cu

Lines 45 to 50 in e5f8dd3

	// The functor returns the index of minimum in the sorted values.
	// We need the index of minimum in the original unsorted values.
	// So use indices to gather the sort order used to sort `values`.
	// The values in data buffer of indices corresponding to null values was
	// initialized to ARGMIN_SENTINEL. Using gather_if.
	// This can't use gather because nulls in gathered column will not store ARGMIN_SENTINEL.

Steps/Code to reproduce bug
Current code will contains null value with mask set and value of 0 (instead of -1). So the following gather_if seems to be unnecessary because the predicate will always be true:

cudf/cpp/src/groupby/sort/group_argmin.cu

Line 58 in e5f8dd3

[] __device__(auto i) { return (i != cudf::detail::ARGMIN_SENTINEL); });

It will also affect the logic of min which uses argmin

cudf/cpp/src/groupby/hash/groupby.cu

Lines 235 to 240 in e5f8dd3

	// We make a view of ARG(MIN/MAX) result without a null mask and gather
	// using this map. The values in data buffer of ARG(MIN/MAX) result
	// corresponding to null values was initialized to ARG(MIN/MAX)_SENTINEL
	// which is an out of bounds index value (-1) and causes the gathered
	// value to be null.
	column_view null_removed_map(

It didn't show any correctness issues under my tests and I don't think it did the wrong thing but the comment is confusing.

Expected behavior
We can remove the comment and update the related code, like using gather instead of gather_if, if the initialization is really not needed. Or if it's a bug we need to fix it.

Additional context
group_argmax also has the same issue.