### What changes were proposed in this pull request?
Fixes a Python UDF `plus_one` used in `GroupedAggPandasUDFTests` to always return float (double) values.
### Why are the changes needed?
The Python UDF `plus_one` used in `GroupedAggPandasUDFTests` is always returning `v + 1` regardless of its type. The return type of the UDF is 'double', so if the input is int, the result will be `null`.
```py
>>> df = spark.range(10).toDF('id') \
... .withColumn("vs", array([lit(i * 1.0) + col('id') for i in range(20, 30)])) \
... .withColumn("v", explode(col('vs'))) \
... .drop('vs') \
... .withColumn('w', lit(1.0))
>>> udf('double')
... def plus_one(v):
... assert isinstance(v, (int, float))
... return v + 1
...
>>> pandas_udf('double', PandasUDFType.GROUPED_AGG)
... def sum_udf(v):
... return v.sum()
...
>>> df.groupby(plus_one(df.id)).agg(sum_udf(df.v)).show()
+------------+----------+
|plus_one(id)|sum_udf(v)|
+------------+----------+
| null| 2900.0|
+------------+----------+
```
This is meaningless and should be:
```py
>>> udf('double')
... def plus_one(v):
... assert isinstance(v, (int, float))
... return float(v + 1)
...
>>> df.groupby(plus_one(df.id)).agg(sum_udf(df.v)).sort('plus_one(id)').show()
+------------+----------+
|plus_one(id)|sum_udf(v)|
+------------+----------+
| 1.0| 245.0|
| 2.0| 255.0|
| 3.0| 265.0|
| 4.0| 275.0|
| 5.0| 285.0|
| 6.0| 295.0|
| 7.0| 305.0|
| 8.0| 315.0|
| 9.0| 325.0|
| 10.0| 335.0|
+------------+----------+
```
### Does this PR introduce _any_ user-facing change?
No, test-only.
### How was this patch tested?
Fixed the test.
Closes#31730 from ueshin/issues/SPARK-34610/test_pandas_udf_grouped_agg.
Authored-by: Takuya UESHIN <ueshin@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>