Add `Llama4TextModel` to `AutoModel` mapping
Author: ringohoffman
Pull request: yes

# What does this PR do?

Using `Llama4TextConfig` on `AutoModel.from_config` raises a `ValueError` when it is expected to instantiate a `Llama4TextModel`. This PR adds `Llama4TextModel` to the `AutoModel` mapping.

```python
import accelerate
import transformers

config = transformers.AutoConfig.from_pretrained("/models/meta-llama/Llama-4-Scout-17B-16E-Instruct").get_text_config(decoder=True)

with accelerate.init_empty_weights():
    model = transformers.AutoModel.from_config(config)  # ValueError: Unrecognized configuration class <class 'transformers.models.llama4.configuration_llama4.Llama4TextConfig'> for this kind of AutoModel: AutoModel.
```

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## Who can review?

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