BUG max_depth=1 should be decision stump in HistGradientBoosti…

[SanthoshBala18]

Reference Issues/PRs

Fixes #16124

What does this implement/fix? Explain your changes.

Modified the validation in TreeGrower to allow max_depth value as 1.

[glemaitre]

You should look at the tests which are failing. I assume that there a lot of places where we check the number of nodes or the depth where the count will be off now.

[thomasjpfan]

There is a few places where the depth is stored with the original definition in mind. For example, the node_struct stores depth based on the original definition.

This needs a test to assert that the predictors are stumps with max_depth==1.

[SanthoshBala18]

@thomasjpfan, the test "test_max_depth" in test_grower.py asserts the tree depth to be 1 when max_depth is set to 1. If we need something more, can you please guide me on what needs to be tested.?

[thomasjpfan]

The nodes stored in the predictor uses the previous definition of depth.

For this PR, one needs to go through the comments and docstrings in the _hist module to make sure it is using the updated definition of depth.

[NicolasHug]

The nodes stored in the predictor uses the previous definition of depth.

I don't think that's true, the predictor nodes use the same convention as the grower node since we just do node['depth'] = grower_node.depth.

However yes, docstrings should be updated

[thomasjpfan]

I don't think that's true, the predictor nodes use the same convention as the grower node since

On master, the root node is set to depth 0 and the children are set according, which means the node's depth was set using the tree-depth definition: "The depth of a node is the length of the path to its root". It looks like it was impossible to create stumps before this PR.

[NicolasHug]

Thanks @SanthoshBala18 , looks good.

Please also udpate the docstrings of TreeGrower and HistGradientBoostingClassifier.

Let's also add a non-regression test to make sure that we have a stump when we pass max_depth=1, with e.g.

def assert_is_stump(grower):
	for leaf in (grower.root.left_child, grower.root.right_child):
		assert leaf.left_child is None
		assert leaf.right_child is None

you can use that in test_max_depth, or create a similar new test.

[NicolasHug]

Suggested change

	nodes to go from the root to the deepest leaf. Depth isn't constrained by default.
	edges to go from the root to the deepest leaf. Depth isn't constrained by default.

[NicolasHug]

Please also update the classifier's docstring

[SanthoshBala18]

@NicolasHug Thanks for the review. I have updated the docstrings and also added a test accordingly.

[NicolasHug]

Thanks @SanthoshBala18 , LGTM

Please add an entry to the change log at doc/whats_new/v0.23.rst. Like the other entries there, please reference this pull request with :issue: and credit yourself with :user:.

[NicolasHug]

thanks @SanthoshBala18

[NicolasHug]

Suggested change

	- |Fix| Fixed HistGradientBoosting to allow max_depth=1
	- |Fix| Changed the convention for `max_depth` parameter of :class:`ensemlble.HistGradientBoostingClassifier` and :class:`ensemlble.HistGradientBoostingRegressor`. The depth now corresponds to the number of edges to go from the root to the deepest leaf. Stumps (trees with one split) are now allowed.

(skip lines where appropriate ;) )

[SanthoshBala18]

Sorry my bad. Would update the changes properly from next time :)

[NicolasHug]

Suggested change

	:class:`ensemlble.HistGradientBoostingClassifier` and
	:class:`ensemble.HistGradientBoostingClassifier` and

same below ;)

[thomasjpfan]

LGTM

[thomasjpfan]

Thank you @SanthoshBala18 for the PR!