[FIX] TextGenerationPipeline is currently broken. (#8256)

* [FIX] TextGenerationPipeline is currently broken.

It's most likely due to #8180.
What's missing is a multi vs single string handler at the beginning of
the pipe.
And also there was no testing of this pipeline.

* Fixing Conversational tests too.

tests/test_pipelines_text_generation.py

@@ -1,5 +1,7 @@
 import unittest
 
+from transformers import pipeline
+
 from .test_pipelines_common import MonoInputPipelineCommonMixin
 
 
@@ -8,3 +10,20 @@ class TextGenerationPipelineTests(MonoInputPipelineCommonMixin, unittest.TestCas
     pipeline_running_kwargs = {"prefix": "This is "}
     small_models = ["sshleifer/tiny-ctrl"]  # Models tested without the @slow decorator
     large_models = []  # Models tested with the @slow decorator
+
+    def test_simple_generation(self):
+        nlp = pipeline(task="text-generation", model=self.small_models[0])
+        # text-generation is non-deterministic by nature, we can't fully test the output
+
+        outputs = nlp("This is a test")
+
+        self.assertEqual(len(outputs), 1)
+        self.assertEqual(list(outputs[0].keys()), ["generated_text"])
+        self.assertEqual(type(outputs[0]["generated_text"]), str)
+
+        outputs = nlp(["This is a test", "This is a second test"])
+        self.assertEqual(len(outputs[0]), 1)
+        self.assertEqual(list(outputs[0][0].keys()), ["generated_text"])
+        self.assertEqual(type(outputs[0][0]["generated_text"]), str)
+        self.assertEqual(list(outputs[1][0].keys()), ["generated_text"])
+        self.assertEqual(type(outputs[1][0]["generated_text"]), str)