Improving Pipelines by defaulting to framework='tf' when pytorch seems unavailable.
Author: Narsil
Pull request: yes

# What does this PR do?

When loading a model that was `tf` only and by passing only model by string without framework argument, it would fail with an odd error message:

```python
>>> transformers.AutoModel.from_pretrained('Narsil/small')
OSError: Can't load weights for 'Narsil/small'. Make sure that:
- 'Narsil/small' is a correct model identifier listed on 'https://huggingface.co/models' (It exists and contains tf_model.h5)
- or 'Narsil/small' is the correct path to a directory containing a file named one of pytorch_model.bin, tf_model.h5, model.ckpt.
```

This PR corrects the `get_framework` that happens very early in the pipeline to detect the type of model
automatically. It does trigger an early download, but that will happen anyway later. 


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@mfuntowicz

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Comments:
Actually, This introduced a pretty big bug where

```python
nlp = pipeline(task) 
```

Would not work anymore. I tried a different solution around that.