Apply Flash-attn for MAE-based models.

[JerryFlymi]

According to the paper：
"Surprisingly, proportional attention is necessary for supervised models (e.g., AugReg, SWAG, DeiT), but not for MAE models."
We can easily arm MAE-based models with Flash attention.

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[dbolya]

Good idea! I don't want to require a new library for ToMe though and I'd rather flash attention be default off to be consistent with the paper.

As for requiring a library, have you tried torch.nn.functional's scaled_dot_product_attention? Is the performance similar to flash attention? Because if so, we should probably use that instead.