Add Exception when flash_attn is not found and ModernBert is used
Author: Darejkal
Pull request: yes

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Fixes #39934

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: modernbert

```
if is_flash_attn_2_available():
    from flash_attn.flash_attn_interface import flash_attn_varlen_qkvpacked_func
    from flash_attn.layers.rotary import RotaryEmbedding
    from flash_attn.ops.triton.rotary import apply_rotary
else:
    RotaryEmbedding = object
```
Hi, `is_flash_attn_2_available()` returns False. It is causing error this line.

```
super().__init__(dim=dim, base=base, device=device, interleaved=False)
``` 

Since the super class is now object, init() takes 0 arguments.

Since it is apparent that the implementation of ModernBertUnpaddedRotaryEmbedding is dependent on RotaryEmbedding, I think it should raise Error during initialization to clarify that flash_attn needs to be installed.

If ` is_flash_attn_2_available()=False`, then the `model.config._attn_implementation` can never be set to FA2 and thus we shouldn't be trying to use FA2 RoPE layer

Are you initiating the model exactly as in the linked issue? 

Yeah, I cannot reproduce this error anymore. And since there is a check in place when attn_implementation is None, I guess this modification is not needed, gonna close the issue. Thank you and sorry for bothering.