Register cudf.core.groupby.Grouper objects to dask `grouper_dispatc…

…h` (rapidsai#10838)

This PR registers uses the (presumably shortly merged) dask `Grouper` dispatch to register `cudf.core.groupby.Grouper` objects to `cudf.DataFrame` objects. This should allow our own Grouper objects to be used in critical places in dask rather than pandas objects. 

This solution is favorable IMO rather than changing cuDF to handle pandas grouper objects directly. 

Xref dask/dask#9074

Authors:
  - https://github.com/brandon-b-miller

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: rapidsai#10838

python/dask_cudf/dask_cudf/backends.py

@@ -296,6 +296,16 @@ def is_categorical_dtype_cudf(obj):
     return cudf.api.types.is_categorical_dtype(obj)
 
 
+try:
+    from dask.dataframe.dispatch import grouper_dispatch
+
+    @grouper_dispatch.register((cudf.Series, cudf.DataFrame))
+    def get_grouper_cudf(obj):
+        return cudf.core.groupby.Grouper
+
+except ImportError:
+    pass
+
 try:
     try:
         from dask.array.dispatch import percentile_lookup