Correctly raise NotImplementedError on dynamic groupbys

Now that the dynamic groupby options are exposed, we can raise without
heuristics.

Partially addresses rapidsai#17144

python/cudf_polars/cudf_polars/dsl/ir.py

@@ -666,11 +666,11 @@ def __init__(
             raise NotImplementedError(
                 "rolling window/groupby"
             )  # pragma: no cover; rollingwindow constructor has already raised
+        if self.options.dynamic:
+            raise NotImplementedError("dynamic group by")
         if any(GroupBy.check_agg(a.value) > 1 for a in self.agg_requests):
             raise NotImplementedError("Nested aggregations in groupby")
         self.agg_infos = [req.collect_agg(depth=0) for req in self.agg_requests]
-        if len(self.keys) == 0:
-            raise NotImplementedError("dynamic groupby")
 
     @staticmethod
     def check_agg(agg: expr.Expr) -> int: