Pipeline for Text Generation: GenerationPipeline (#3758)

* Add GenerationPipeline

* Fix parameter names

* Correct parameter __call__ parameters

* Add model type attribute and correct function calls for prepare_input

* Take out trailing commas from init attributes

* Remove unnecessary tokenization line

* Implement support for multiple text inputs

* Apply generation support for multiple input text prompts

* Take out tensor coersion

* Take out batch index

* Add text prompt to return sequence

* Squeeze token tensore before decoding

* Return only a single list of sequences if only one prompt was used

* Correct results variable name

* Add GenerationPipeline to SUPPORTED_TASKS with the alias , initalized w GPT2

* Registedred AutoModelWithLMHead for both pt and t

* Update docstring for GenerationPipeline

* Add kwargs parameter to mode.generate

* Take out kwargs parameter after all

* Add generation pipeline example in pipeline docstring

* Fix max length by squeezing tokens tensor

* Apply ensure_tensor_on_device to pytorch tensor

* Include generation step in torch.no_grad

* Take out input from prepare_xlm_input and set 'en' as default xlm_language

* Apply framework specific encoding during prepare_input

* Format w make style

* Move GenerationPipeline import to follow proper import sorting

* Take out training comma from generation dict

* Apply requested changes

* Change name to TextGenerationPipeline

* Apply TextGenerationPipeline rename to __init___

* Changing alias to

* Set input mapping as input to ensure_tensor_on_device

* Fix assertion placement

* Add test_text_generation

* Add TextGenerationPipeline to PipelineCommonTests

* Take out whitespace

* Format __init__ w black

* Fix __init__ style

* Forman __init___

* Add line to end of __init__

* Correct model tokenizer set for test_text_generation

* Ensure to return list of list, not list of string (to pass test)

* Limit test models to only 3 to limit runtime to address circleCI timeout error

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update tests/test_pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Remove argument docstring, __init__, add additional __call__ arguments, and reformat results to list of dict

* Fix blank result list

* Add TextGenerationPipeline to pipelines.rst

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Fix typos from adding PADDING_TEXT_TOKEN_LENGTH

* Fix incorrectly moved result list

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

* Update src/transformers/pipelines.py

Co-Authored-By: Patrick von Platen <patrick.v.platen@gmail.com>

* Add back generation line and make style

* Take out blank whitespace

* Apply new alis, text-generation, to test_pipelines

* Fix text generation alias in test

* Update src/transformers/pipelines.py

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Julien Chaumond <chaumond@gmail.com>

tests/test_pipelines.py

@@ -60,6 +60,11 @@ TEXT_CLASSIF_FINETUNED_MODELS = {
     )
 }
 
+TEXT_GENERATION_FINETUNED_MODELS = {
+    ("gpt2", "gpt2"),
+    ("xlnet-base-cased", "xlnet-base-cased"),
+}
+
 FILL_MASK_FINETUNED_MODELS = [
     (("distilroberta-base", {"use_fast": False}), "distilroberta-base", None),
 ]
@@ -293,6 +298,16 @@ class MonoColumnInputTestCase(unittest.TestCase):
                 nlp, valid_inputs, invalid_inputs, mandatory_keys,
             )
 
+    @require_torch
+    def test_text_generation(self):
+        valid_inputs = ["A string like this", ["list of strings entry 1", "list of strings v2"]]
+        invalid_inputs = [None]
+        for model, tokenizer in TEXT_GENERATION_FINETUNED_MODELS:
+            nlp = pipeline(task="text-generation", model=model, tokenizer=tokenizer, framework="pt")
+            self._test_mono_column_pipeline(
+                nlp, valid_inputs, invalid_inputs, {},
+            )
+
 
 class MultiColumnInputTestCase(unittest.TestCase):
     def _test_multicolumn_pipeline(self, nlp, valid_inputs: list, invalid_inputs: list, output_keys: Iterable[str]):
@@ -371,6 +386,7 @@ class PipelineCommonTests(unittest.TestCase):
         "translation_en_to_fr",
         "translation_en_to_de",
         "translation_en_to_ro",
+        "text-generation",
     )
 
     @slow