Purge unused ModelTester code (#37085)

* Purge correctly this time

* Remove more methods from recent PRs

* make fixup

tests/models/umt5/test_modeling_umt5.py

@@ -229,43 +229,6 @@ class UMT5ModelTester:
         # There should be a self attn key, a self attn value, a cross attn key and a cross attn value stored in each decoder_past tuple
         self.parent.assertEqual(len(decoder_past[0]), 4)
 
-    def create_and_check_decoder_model_past(
-        self,
-        config,
-        input_ids,
-        decoder_input_ids,
-        attention_mask,
-        decoder_attention_mask,
-        lm_labels,
-    ):
-        model = UMT5Model(config=config).get_decoder().to(torch_device).eval()
-        # first forward pass
-        outputs = model(input_ids, use_cache=True)
-        outputs_use_cache_conf = model(input_ids)
-        outputs_no_past = model(input_ids, use_cache=False)
-
-        self.parent.assertTrue(len(outputs) == len(outputs_use_cache_conf))
-        self.parent.assertTrue(len(outputs) == len(outputs_no_past) + 1)
-
-        output, past_key_values = outputs.to_tuple()
-
-        # create hypothetical next token and extent to next_input_ids
-        next_tokens = ids_tensor((self.batch_size, 1), config.vocab_size)
-
-        # append to next input_ids and
-        next_input_ids = torch.cat([input_ids, next_tokens], dim=-1)
-
-        output_from_no_past = model(next_input_ids)["last_hidden_state"]
-        output_from_past = model(next_tokens, past_key_values=past_key_values)["last_hidden_state"]
-
-        # select random slice
-        random_slice_idx = ids_tensor((1,), output_from_past.shape[-1]).item()
-        output_from_no_past_slice = output_from_no_past[:, -1, random_slice_idx].detach()
-        output_from_past_slice = output_from_past[:, 0, random_slice_idx].detach()
-
-        # test that outputs are equal for slice
-        self.parent.assertTrue(torch.allclose(output_from_past_slice, output_from_no_past_slice, atol=1e-3))
-
     def create_and_check_model_fp16_forward(
         self,
         config,