[generate] shape checks in tests compatible with fixed-length caches (+ some minor fixes) (#35993)

* shape checks compatible with static cache

* add test

* tmp

* manually turn on eager attn when we want to output attn

* typo

* generalize to encoder-decoder models

* force compilation on cpu

* tmp commit

* fix static cache shape checks

* models with odd caches

* fix copies

* shorter cache search loop

* use decoder_past_key_values everywhere

* better test variable names and comments

* signature

* rename _check_outputs into _check_generate_outputs

* add comments

* HybridCache future test note

tests/models/imagegpt/test_modeling_imagegpt.py

@@ -251,10 +251,10 @@ class ImageGPTModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterM
         return inputs_dict
 
     # we overwrite the _check_scores method of GenerationTesterMixin, as ImageGPTForCausalImageModeling doesn't have tied input- and output embeddings
-    def _check_scores(self, batch_size, scores, length, config):
+    def _check_scores(self, batch_size, scores, generated_length, config):
         expected_shape = (batch_size, config.vocab_size - 1)
         self.assertIsInstance(scores, tuple)
-        self.assertEqual(len(scores), length)
+        self.assertEqual(len(scores), generated_length)
         self.assertListEqual([iter_scores.shape for iter_scores in scores], [expected_shape] * len(scores))
 
     @run_test_using_subprocess