Support DecimalDtype meta in dask_cudf (#16634)

To enable some tpch benchmarking for dask-cudf

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

Approvers:
  - Richard (Rick) Zamora (https://github.com/rjzamora)

URL: #16634

python/dask_cudf/dask_cudf/backends.py

@@ -134,6 +134,8 @@ def _get_non_empty_data(
         return cudf.core.column.as_column(
             np.arange(start=0, stop=2, dtype=s.dtype)
         )
+    elif isinstance(s.dtype, cudf.core.dtypes.DecimalDtype):
+        return cudf.core.column.as_column(range(2), dtype=s.dtype)
     else:
         raise TypeError(
             f"Don't know how to handle column of type {type(s).__name__}"

python/dask_cudf/dask_cudf/tests/test_join.py

@@ -386,3 +386,14 @@ def test_issue_12773():
         expected.to_pandas(),
         check_index=False,
     )
+
+
+@pytest.mark.parametrize(
+    "typ", [cudf.Decimal32Dtype, cudf.Decimal64Dtype, cudf.Decimal128Dtype]
+)
+def test_merge_on_decimal(typ):
+    df = cudf.DataFrame({"a": [1], "b": [2]}, dtype=typ(1))
+    ddf = dask_cudf.from_cudf(df, npartitions=1)
+    result = ddf.merge(ddf, left_on="a", right_on="a")
+    expected = df.merge(df, left_on="a", right_on="a")
+    dd.assert_eq(result, expected)