Add Flex Attention for Mistral along with refactoring
Author: OmarManzoor
Pull request: yes

# What does this PR do?

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Towards #34809

- Adds Flex Attention for Mistral
- Does refactoring to enable the attention mechanisms using functions instead of classes


## Who can review?

@ArthurZucker 

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Comments:
@ArthurZucker What should be do about these failing tests? I think they are related to the sdpa tests where we might have `output_attentions` equal to `True`. 

Hi. Let's take a look on failing test(s) step by step.

First, do you know why `Idefics2` will have something `MistralAttention`? It's very strange. (see the `test_torch job` log)

> FAILED tests/models/idefics2/test_modeling_idefics2.py::Idefics2ForConditionalGenerationModelTest::test_retain_grad_hidden_states_attentions - AttributeError: 'MistralAttention' object has no attribute 'scaling'

> Hi. Let's take a look on failing test(s) step by step.
> 
> First, do you know why `Idefics2` will have something `MistralAttention`? It's very strange. (see the `test_torch job` log)
> 
> > FAILED tests/models/idefics2/test_modeling_idefics2.py::Idefics2ForConditionalGenerationModelTest::test_retain_grad_hidden_states_attentions - AttributeError: 'MistralAttention' object has no attribute 'scaling'

Idefics2 uses mistral as the text model
```python
self.text_model = AutoModel.from_config(config.text_config)
```

```python
        if isinstance(text_config, dict):
            text_config["model_type"] = text_config["model_type"] if "model_type" in text_config else "mistral"
            text_config = CONFIG_MAPPING[text_config["model_type"]](**text_config)
        elif text_config is None:
            logger.info("text_config is None, using default text config")
            text_config = CONFIG_MAPPING["mistral"](
                max_position_embeddings=4096 * 8,
                rms_norm_eps=1e-5,
                # None in the original configuration_mistral, we set it to the unk_token_id
                pad_token_id=0,
                tie_word_embeddings=False,
            )
```

BTW let's make sure we rebase now that #34896 was merged!

can you make sure the CIs are green? 🤗 

> can you make sure the CIs are green? 🤗

Should I reset the default back to eager instead of flex because the eager matches sdpa fails for float32 when using flex. Or do we need to change the thresholds to ensure that flex remains the default while the tests are also green?

> Hey! We actually shipped this in #35235 ! 🤗 super sorry for the late notice

Thanks for informing.