style: make AggregateFnV2 generic over accumulator/result (Generic[AggType, U])

Signed-off-by: Arthur <atte.book@gmail.com>

Author: ArthurBook

python/ray/data/aggregate.py

@@ -1,6 +1,6 @@
 import abc
 import math
-from typing import TYPE_CHECKING, Any, Callable, List, Optional
+from typing import TYPE_CHECKING, Any, Callable, Generic, List, Optional
 
 import numpy as np
 import pyarrow.compute as pc
@@ -113,7 +113,7 @@ def _validate(self, schema: Optional["Schema"]) -> None:
 
 
 @PublicAPI(stability="alpha")
-class AggregateFnV2(AggregateFn, abc.ABC):
+class AggregateFnV2(AggregateFn, abc.ABC, Generic[AggType, U]):
     """Provides an interface to implement efficient aggregations to be applied
     to the dataset.