[FEA]need support 'product' method in cudf.Dataframe.groupby.agg

[pengsnv]

When I try to use cudf.DataFrame.groupby.agg with 'product' method, it report an error.
steps to reproduce the error:

1. start the container:
   nvidia-docker run -it --rm nvcr.io/nvidia/rapidsai/rapidsai:cuda10.1-runtime-ubuntu18.04
2. run the following code:
   import cudf data=cudf.DataFrame({'a':[1,1,2],'b':[2,3,4]}) data.groupby('a').agg('product')
3. the error is:
   Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/opt/conda/envs/rapids/lib/python3.6/site-packages/cudf/core/groupby/groupby.py", line 143, in agg result = self._groupby.aggregate(self.obj, normalized_aggs) File "cudf/_libxx/groupby.pyx", line 144, in cudf._libxx.groupby.GroupBy.aggregate RuntimeError: cuDF failure at: /conda/conda-bld/libcudf_1585640494801/work/cpp/src/aggregation/result_cache.cpp:88: Result does not exist in cache
   I find cudf.Dataframe.groupby.agg support 'product' in the source code aggregation.pyx , but it is not listed ingroupby.pyx

This is an important method in my customer's procedure. Can we support this method?

[github-actions]

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[github-actions]

This issue has been labeled inactive-30d due to no recent activity in the past 30 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed. This issue will be labeled inactive-90d if there is no activity in the next 60 days.