Fixing t2t pipelines lists outputs. (#15008)

Backward compatibility broken in
https://github.com/huggingface/transformers/pull/14988

src/transformers/pipelines/text2text_generation.py

@@ -136,8 +136,8 @@ class Text2TextGenerationPipeline(Pipeline):
         """
 
         result = super().__call__(*args, **kwargs)
-        if isinstance(result, dict):
-            return [result]
+        if isinstance(args[0], list) and all(isinstance(el, str) for el in args[0]):
+            return [res[0] for res in result]
         return result
 
     def preprocess(self, inputs, truncation=TruncationStrategy.DO_NOT_TRUNCATE, **kwargs):

tests/test_pipelines_translation.py

@@ -47,6 +47,12 @@ class TranslationPipelineTests(unittest.TestCase, metaclass=PipelineTestCaseMeta
         outputs = translator("Some string")
         self.assertEqual(outputs, [{"translation_text": ANY(str)}])
 
+        outputs = translator(["Some string"])
+        self.assertEqual(outputs, [{"translation_text": ANY(str)}])
+
+        outputs = translator(["Some string", "other string"])
+        self.assertEqual(outputs, [{"translation_text": ANY(str)}, {"translation_text": ANY(str)}])
+
     @require_torch
     def test_small_model_pt(self):
         translator = pipeline("translation_en_to_ro", model="patrickvonplaten/t5-tiny-random", framework="pt")