cannot import name 'Text2TextGenerationPipeline
In transformers 4.17，It will find error like 
："cannot import name 'Text2TextGenerationPipeline'"

My code is  "from transformers import Text2TextGenerationPipeline"
The recommended way to load the `Text2TextGenerationPipeline` as stated in the [docs](https://huggingface.co/docs/transformers/v4.17.0/en/main_classes/pipelines#transformers.Text2TextGenerationPipeline) is:

```python
from transformers import pipeline
text2text = pipeline("text2text-generation") 
```

But if you must import the class you can import it as such:
```python
from transformers.pipelines.text2text_generation import Text2TextGenerationPipeline
```

If you're just trying to use the pipeline and not making something like a feature contribution for the class, you should use the first method to load the pipeline.

```python
class Text2TextGenerationPipeline(Pipeline):
    """
    Pipeline for text to text generation using seq2seq models.
    This Text2TextGenerationPipeline pipeline can currently be loaded from [`pipeline`] using the following task
    identifier: `"text2text-generation"`.
    The models that this pipeline can use are models that have been fine-tuned on a translation task. See the
    up-to-date list of available models on
    [huggingface.co/models](https://huggingface.co/models?filter=text2text-generation).
    Usage:
    ```python
    text2text_generator = pipeline("text2text-generation")
    text2text_generator("question: What is 42 ? context: 42 is the answer to life, the universe and everything")
    ```"""

    # Used in the return key of the pipeline.
    return_name = "generated"

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        self.check_model_type(
            TF_MODEL_FOR_SEQ_TO_SEQ_CAUSAL_LM_MAPPING
            if self.framework == "tf"
            else MODEL_FOR_SEQ_TO_SEQ_CAUSAL_LM_MAPPING
        )
```
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