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/bart/test_modeling_bart.py

@@ -507,6 +507,10 @@ class BartModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin
         model.generate(input_ids, attention_mask=attention_mask)
         model.generate(num_beams=4, do_sample=True, early_stopping=False, num_return_sequences=3)
 
+    @unittest.skip("Does not support conversations.")
+    def test_pipeline_conversational(self):
+        pass
+
 
 def assert_tensors_close(a, b, atol=1e-12, prefix=""):
     """If tensors have different shapes, different values or a and b are not both tensors, raise a nice Assertion error."""