Conversation pipeline fixes (#26795)

* Adjust length limits and allow naked conversation list inputs

* Adjust length limits and allow naked conversation list inputs

* Maybe use a slightly more reasonable limit than 1024

* Skip tests for old models that never supported this anyway

* Cleanup input docstrings

* More docstring cleanup + skip failing TF test

* Make fixup

tests/models/t5/test_modeling_t5.py

@@ -891,6 +891,10 @@ class T5ModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin,
     def test_disk_offload(self):
         pass
 
+    @unittest.skip("Does not support conversations.")
+    def test_pipeline_conversational(self):
+        pass
+
 
 class T5EncoderOnlyModelTester:
     def __init__(