Update model implementations to use flash attention

[divyashreepathihalli]

Flash attention support has been added to Keras 3.
https://github.com/keras-team/keras/blob/25d6d80a6ecd31f0da52c325cd16dbe4a29b7329/keras/src/layers/attention/multi_head_attention.py#L55

However, some of the models implemented in KerasHub is overriding def _compute_attention() function which has the flash attention enabling mechanism. The implementations need to be updated

[shanky-kapoor]

@divyashreepathihalli Is this flash attention 2? I have a bug assigned here. If this is already done we can close that. If not I can start working on top of this.