[FIX] TextGenerationPipeline is currently broken.
Author: Narsil
Pull request: yes

# What does this PR do?

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.

This also changes Conversational pipeline tests which seemed to have also test failures
This was linked to having state within the input that get consumed and the tests did not recreate
them so we had a stale `Conversation` object for the new test.


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Comments:
Ran all pipeline tests with @slow too to make sure:

```
==================================================================== warnings summary ====================================================================
.venv/lib/python3.8/site-packages/tensorflow/python/autograph/utils/testing.py:21
  /home/nicolas/src/transformers/.venv/lib/python3.8/site-packages/tensorflow/python/autograph/utils/testing.py:21: DeprecationWarning: the imp module is 
deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

tests/test_pipelines_fill_mask.py::FillMaskPipelineTests::test_tf_fill_mask_results
  /home/nicolas/src/transformers/src/transformers/pipelines.py:1200: FutureWarning: The `topk` argument is deprecated and will be removed in a future vers
ion, use `top_k` instead.
    warnings.warn(

-- Docs: https://docs.pytest.org/en/stable/warnings.html
====================================================== 93 passed, 2 warnings in 2234.37s (0:37:14) =======================================================

```