Add post_traversal API to cudf-polars

[rjzamora]

Description

• Adds post_traversal API.
• Copied from @wence-'s wence-:more-tablestat-doodles branch.
• Used in [WIP] Track column statistics in cuDF-Polars #19130 (creating this separate API to simplify code review)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[TomAugspurger]

Looks good, thanks.

Only question is around test coverage. It's probably worth having a a test with multiple nodes to ensure that we go through the expressions in the right order (I suspect that's important). I'd suggest something, but I'm not sure offhand whether it goes through the first or last node first 😓

[TomAugspurger]

This maybe?

diff --git a/python/cudf_polars/tests/dsl/test_traversal.py b/python/cudf_polars/tests/dsl/test_traversal.py
index 7b7312fdd0..1b95e04b36 100644
--- a/python/cudf_polars/tests/dsl/test_traversal.py
+++ b/python/cudf_polars/tests/dsl/test_traversal.py
@@ -71,6 +71,24 @@ def test_post_traversal_unique():
     assert unique_exprs == [expr.Col(dt, "b"), expr.Col(dt, "a"), e3]
 
 
+def test_post_traversal_multi():
+    dt = DataType(pl.datatypes.Int8())
+
+    e1 = make_expr(dt, "a", "a")
+    e2 = make_expr(dt, "a", "b")
+    e3 = make_expr(dt, "b", "a")
+
+    unique_exprs = list(post_traversal([e1, e2, e3]))
+    assert len(unique_exprs) == 5
+    assert unique_exprs == [
+        expr.Col(dt, "b"),
+        expr.Col(dt, "a"),
+        e3,
+        e2,
+        e1,
+    ]
+
+

That passes. Hopefully it's correct.

[rjzamora]

That passes. Hopefully it's correct.

Thanks @TomAugspurger ! Seems reasonable to me (added).

[TomAugspurger]

Fixed the merge conflict from #19135.

[TomAugspurger]

/merge