[ConvBert] Fix #21523 (#21849)

* fix reshaping
Fixes #21523

* add test

* styling

* last fixes

* Update src/transformers/models/convbert/modeling_convbert.py

* code quallity

tests/models/convbert/test_modeling_convbert.py

@@ -459,6 +459,11 @@ class ConvBertModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCase
         result = model(inputs_embeds=inputs_embeds)
         self.assertEqual(result.last_hidden_state.shape, (batch_size, seq_length, config.hidden_size))
 
+    def test_reducing_attention_heads(self):
+        config, *inputs_dict = self.model_tester.prepare_config_and_inputs()
+        config.head_ratio = 4
+        self.model_tester.create_and_check_for_masked_lm(config, *inputs_dict)
+
 
 @require_torch
 class ConvBertModelIntegrationTest(unittest.TestCase):