[BUG] cudf::hash_join compute size fails on struct columns

[jlowe]

Describe the bug
Using cudf::hash_join to calculate the output row count for a join involving struct columns results in the following exception being thrown.

cuDF failure at: ../include/cudf/table/row_operators.cuh:231: Mismatched number of columns.

It looks like the build table is flattened when the hash is built but the probe table isn't also flattened when computing the output size.

Steps/Code to reproduce bug
Apply the following patch and run the libcudf join tests.

diff --git a/cpp/tests/join/join_tests.cpp b/cpp/tests/join/join_tests.cpp
index e468368842..22bf7aa2ff 100644
--- a/cpp/tests/join/join_tests.cpp
+++ b/cpp/tests/join/join_tests.cpp
@@ -501,6 +501,50 @@ TEST_F(JoinTest, LeftJoinWithStructsAndNulls)
   CUDF_TEST_EXPECT_TABLES_EQUIVALENT(*sorted_gold, *sorted_result);
 }
 
+TEST_F(JoinTest, LeftHashJoinWithStructsAndNulls)
+{
+  column_wrapper<int32_t> col0_0{{3, 1, 2, 0, 2}};
+  strcol_wrapper col0_1({"s1", "s1", "", "s4", "s0"}, {1, 1, 0, 1, 1});
+  column_wrapper<int32_t> col0_2{{0, 1, 2, 4, 1}};
+  auto col0_names_col = strcol_wrapper{
+    "Samuel Vimes", "Carrot Ironfoundersson", "Detritus", "Samuel Vimes", "Angua von Überwald"};
+  auto col0_ages_col = column_wrapper<int32_t>{{48, 27, 351, 31, 25}};
+
+  auto col0_is_human_col = column_wrapper<bool>{{true, true, false, false, false}, {1, 1, 0, 1, 0}};
+
+  auto col0_3 =
+    cudf::test::structs_column_wrapper{{col0_names_col, col0_ages_col, col0_is_human_col}};
+
+  column_wrapper<int32_t> col1_0{{2, 2, 0, 4, 3}};
+  strcol_wrapper col1_1({"s1", "s0", "s1", "s2", "s1"});
+  column_wrapper<int32_t> col1_2{{1, 0, 1, 2, 1}, {1, 0, 1, 1, 1}};
+  auto col1_names_col = strcol_wrapper{
+    "Samuel Vimes", "Detritus", "Detritus", "Carrot Ironfoundersson", "Angua von Überwald"};
+  auto col1_ages_col = column_wrapper<int32_t>{{48, 35, 351, 22, 25}};
+
+  auto col1_is_human_col = column_wrapper<bool>{{true, true, false, false, true}, {1, 1, 0, 1, 1}};
+
+  auto col1_3 =
+    cudf::test::structs_column_wrapper{{col1_names_col, col1_ages_col, col1_is_human_col}};
+
+  CVector cols0, cols1;
+  cols0.push_back(col0_0.release());
+  cols0.push_back(col0_1.release());
+  cols0.push_back(col0_2.release());
+  cols0.push_back(col0_3.release());
+  cols1.push_back(col1_0.release());
+  cols1.push_back(col1_1.release());
+  cols1.push_back(col1_2.release());
+  cols1.push_back(col1_3.release());
+
+  Table t0(std::move(cols0));
+  Table t1(std::move(cols1));
+
+  cudf::hash_join hash_join(t1, cudf::null_equality::EQUAL);
+  auto output_size = hash_join.left_join_size(t0, cudf::null_equality::EQUAL);
+  EXPECT_EQ(output_size, 5);
+}
+
 TEST_F(JoinTest, LeftJoinOnNulls)
 {
   // clang-format off

Expected behavior
The test above should pass.