fix(pipeline): error when model not in AutoModel
Author: voidful
Pull request: yes


# What does this PR do?

During pipeline initialize, it will call get_framework to check whether tf or pt model. get_framework will call AutoModel to load model and return where it depends on.

However, not all the models are in AutoModel. For example, `Helsinki-NLP/opus-mt-en-fr` is under AutoModelForSeq2SeqLM which will cause an error when calling pipeline.

get_framework should depend on task instead. This fix pass targeted_task
to get_framework,  if the model not in AutoModel, it will use the targeted_task model instead.

error example
```
from transformers import pipeline
model = pipeline(task="translation_en_to_fr",model="Helsinki-NLP/opus-mt-en-fr")
```

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

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