[generation] bring back tests on vision models (#38603)

* bring back geenration tests on VLMs

* remove head mask tests overwritten

tests/models/blip_2/test_modeling_blip_2.py

@@ -774,6 +774,7 @@ class Blip2TextModelTester:
             bos_token_id=self.pad_token_id,
             pad_token_id=self.pad_token_id,
             decoder_start_token_id=self.decoder_start_token_id,
+            is_encoder_decoder=True,
         )
 
 
@@ -795,6 +796,9 @@ class Blip2ModelTester:
         self.text_model_tester = Blip2TextModelTester(parent, **text_kwargs)
         self.batch_size = self.text_model_tester.batch_size  # need bs for batching_equivalence test
         self.seq_length = self.text_model_tester.seq_length  # need seq_length for common tests
+        self.encoder_seq_length = (
+            self.text_model_tester.encoder_seq_length + num_query_tokens
+        )  # need enc seq_length for gen tests
         self.is_training = is_training
         self.num_query_tokens = num_query_tokens
 
@@ -859,11 +863,9 @@ class Blip2ModelTester:
 
 
 @require_torch
-class Blip2ModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCase):
+class Blip2ModelTest(ModelTesterMixin, PipelineTesterMixin, GenerationTesterMixin, unittest.TestCase):
     all_model_classes = (Blip2ForConditionalGeneration, Blip2Model) if is_torch_available() else ()
     additional_model_inputs = ["input_ids", "decoder_input_ids"]
-    # Doesn't run generation tests. TODO: fix generation tests for Blip2ForConditionalGeneration
-    all_generative_model_classes = ()
     pipeline_model_mapping = (
         {
             "feature-extraction": Blip2Model,