RowGroupColumnName of ranking trainers options class defaults to null

[artidoro]

The Options class of the ranking trainers (FastTree and LightGbm) defaults to RowGroupColumnName = null.

This is:

1. Inconsistent with the simple constructor where RowGroupColumnName defaults to GroupId
2. Not desirable as in ranking the row group is very important for correct training

Here are the lines where the row group column name is set:

machinelearning/src/Microsoft.ML.Data/Training/TrainerInputBase.cs

Lines 106 to 110 in 738e5d5

	/// <summary>
	/// Column to use for example groupId.
	/// </summary>
	[Argument(ArgumentType.AtMostOnce, HelpText = "Column to use for example groupId", ShortName = "groupId", SortOrder = 5, Visibility = ArgumentAttribute.VisibilityType.EntryPointsOnly)]
	public string RowGroupColumnName = null;

We need to update the default and align it with the simple constructor.

[artidoro]

I am afraid that this will require a breaking API change which at this point we can't do. This would be something that we should consider changing for 2.0.

[justinormont]

I would expect the ranking trainer to throw if it doesn't have a RowGroupColumnName defined.

Hence, I don't see this as a breaking change, since without it the user's pipeline (even if it runs) is broken as the result won't have utility. This questions our definition of a breaking API change.

[najeeb-kazmi]

Tracking in #4749.