[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/longt5/test_modeling_longt5.py

@@ -785,7 +785,7 @@ class LongT5ModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMix
                     [self.model_tester.num_attention_heads, block_len, 3 * block_len],
                 )
 
-    def _check_encoder_attention_for_generate(self, attentions, batch_size, config, seq_length):
+    def _check_encoder_attention_for_generate(self, attentions, batch_size, config, prompt_length):
         block_len = getattr(self.model_tester, "block_len", None)
         encoder_expected_shape = (batch_size, 2, config.num_attention_heads, block_len, 3 * block_len)
         self.assertIsInstance(attentions, tuple)
@@ -920,10 +920,10 @@ class LongT5TGlobalModelTest(LongT5ModelTest):
                     [self.model_tester.num_attention_heads, block_len, 3 * block_len + global_seq_len],
                 )
 
-    def _check_encoder_attention_for_generate(self, attentions, batch_size, config, seq_length):
+    def _check_encoder_attention_for_generate(self, attentions, batch_size, config, prompt_length):
         block_len = getattr(self.model_tester, "block_len", None)
         global_block_size = getattr(self.model_tester, "global_block_size", None)
-        global_seq_length = seq_length // global_block_size
+        global_seq_length = prompt_length // global_block_size
         encoder_expected_shape = (
             batch_size,
             2,