pandas API on Spark: DataFrameGroupBy.apply raises an exception when it returns Series.

>>> ps.options.compute.shortcut_limit = 3
>>> psdf = ps.DataFrame(
...     {"a": [1, 2, 3, 4, 5, 6], "b": [1, 1, 2, 3, 5, 8], "c": [1, 4, 9, 16, 25, 36]},
...     columns=["a", "b", "c"],
... )
>>> psdf.groupby("b").apply(lambda x: x["a"])
org.apache.spark.api.python.PythonException: Traceback (most recent call last):
...
ValueError: Length mismatch: Expected axis has 2 elements, new values have 3 elements