Fixing return type tensor with num_return_sequences>1. (#16828)

* Fixing return type tensor with `num_return_sequences>1`.

* Nit.

tests/pipelines/test_pipelines_text_generation.py

@@ -56,6 +56,37 @@ class TextGenerationPipelineTests(unittest.TestCase, metaclass=PipelineTestCaseM
             ],
         )
 
+        outputs = text_generator("This is a test", do_sample=True, num_return_sequences=2, return_tensors=True)
+        self.assertEqual(
+            outputs,
+            [
+                {"generated_token_ids": ANY(list)},
+                {"generated_token_ids": ANY(list)},
+            ],
+        )
+        text_generator.tokenizer.pad_token_id = text_generator.model.config.eos_token_id
+        text_generator.tokenizer.pad_token = "<pad>"
+        outputs = text_generator(
+            ["This is a test", "This is a second test"],
+            do_sample=True,
+            num_return_sequences=2,
+            batch_size=2,
+            return_tensors=True,
+        )
+        self.assertEqual(
+            outputs,
+            [
+                [
+                    {"generated_token_ids": ANY(list)},
+                    {"generated_token_ids": ANY(list)},
+                ],
+                [
+                    {"generated_token_ids": ANY(list)},
+                    {"generated_token_ids": ANY(list)},
+                ],
+            ],
+        )
+
     @require_tf
     def test_small_model_tf(self):
         text_generator = pipeline(task="text-generation", model="sshleifer/tiny-ctrl", framework="tf")