Mark test_constrained_beam_search_generate as flaky (#28757)

* Make test_constrained_beam_search_generate as flaky

* Update tests/generation/test_utils.py

tests/generation/test_utils.py

@@ -1293,6 +1293,8 @@ class GenerationTesterMixin:
                     output, input_ids, model.config, num_return_sequences=num_return_sequences * beam_scorer.num_beams
                 )
 
+    # TODO: @gante
+    @is_flaky()
     def test_constrained_beam_search_generate(self):
         for model_class in self.all_generative_model_classes:
             config, input_ids, attention_mask, max_length = self._get_input_ids_and_config()