Avoid realizing a potentially very large RangeIndex in to memory

xref #428

Author: dcherian

Diff for: flox/core.py

@@ -1730,7 +1730,7 @@ def dask_groupby_agg(
                 group_chunks = ((np.nan,),)
             else:
                 assert expected_groups is not None
-                groups = (expected_groups.to_numpy(),)
+                groups = (expected_groups,)
                 group_chunks = ((len(expected_groups),),)
 
         elif method == "cohorts":
@@ -1964,7 +1964,7 @@ def _groupby_aggregate(a, **kwargs):
             num_groups=num_groups,
         )
 
-        groups = (expected_groups.to_numpy(),)
+        groups = (expected_groups,)
 
         return (result, groups)