fixing BC in `fill-mask` (wasn't tested in theses test suites (#13540)

apparently).

src/transformers/pipelines/fill_mask.py

@@ -219,4 +219,7 @@ class FillMaskPipeline(Pipeline):
             - **token** (:obj:`int`) -- The predicted token id (to replace the masked one).
             - **token** (:obj:`str`) -- The predicted token (to replace the masked one).
         """
-        return super().__call__(inputs, **kwargs)
+        outputs = super().__call__(inputs, **kwargs)
+        if isinstance(inputs, list) and len(inputs) == 1:
+            return outputs[0]
+        return outputs

tests/test_pipelines_fill_mask.py

@@ -186,6 +186,18 @@ class FillMaskPipelineTests(unittest.TestCase, metaclass=PipelineTestCaseMeta):
             ],
         )
 
+        outputs = fill_masker([f"This is a {tokenizer.mask_token}"])
+        self.assertEqual(
+            outputs,
+            [
+                {"sequence": ANY(str), "score": ANY(float), "token": ANY(int), "token_str": ANY(str)},
+                {"sequence": ANY(str), "score": ANY(float), "token": ANY(int), "token_str": ANY(str)},
+                {"sequence": ANY(str), "score": ANY(float), "token": ANY(int), "token_str": ANY(str)},
+                {"sequence": ANY(str), "score": ANY(float), "token": ANY(int), "token_str": ANY(str)},
+                {"sequence": ANY(str), "score": ANY(float), "token": ANY(int), "token_str": ANY(str)},
+            ],
+        )
+
         outputs = fill_masker([f"This is a {tokenizer.mask_token}", f"Another {tokenizer.mask_token} great test."])
         self.assertEqual(
             outputs,