Enable ONNX export when PyTorch and TensorFlow installed in the same env
# What does this PR do?

This PR implements one of the proposals discussed in #15620 to enable the ONNX export of pure TensorFlow models (i.e. model repos without PyTorch weights) when TensorFlow and PyTorch are installed in the same environment.

When both frameworks are installed, we default to using an `AutoModel` class (instead of `TFAutoModel`), and then use the `from_tf=True` argument of the `from_pretrained()` method to load the TensorFlow weights.

With this PR, it is now possible to export models like the following without error:

```bash
python -m transformers.onnx --model=keras-io/transformers-qa onnx/
```

The user will see a 404 error in the logs from the `from_pretrained()` method:

```
404 Client Error: Entry Not Found for url: https://huggingface.co/keras-io/transformers-qa/resolve/main/pytorch_model.bin
```

but we can handle this in a separate PR if users report confusion.

Closes #15620 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the reviews 😍 !<|||||>> 404 Client Error: Entry Not Found for url: https://huggingface.co/keras-io/transformers-qa/resolve/main/pytorch_model.bin
> but we can handle this in a separate PR if users report confusion.

Well, I'm reporting this right away. We need to design a way not to report misleading and alarming info and not deal with it in a reactive way. We already have too much logging noise so this just makes things worse. And most users won't report this and just be worried.

Thanks.<|||||>Thanks for the feedback @stas00 - the 404 error is also displayed in the `pipeline()` function with e.g.

```python
from transformers import pipeline

# Load a pure TensorFlow model => see 404 Client Error in logs, but pipeline loads fine
p = pipeline("question-answering", model="keras-io/transformers-qa")
```

I'm happy to take a look at both cases and propose a fix
<|||||>That would be very awesome of you, Lewis! Thank you!