Slimming down torchchat: Replace replace_attention_with_custom_sdpa_attention() with ET's implementation

[Jack-Khuu]

🚀 The feature, motivation and pitch

First surfaced in https://github.com/pytorch/torchchat/pull/1057, the replace_attention_with_custom_sdpa_attention function, used when exporting models in torchchat, can be replaced with the equivalent API provided in the Excecutorch https://github.com/pytorch/executorch/blob/main/examples/models/llama2/source_transformation/sdpa.py

Task: Swap the torchchat implementation with that of ExecuTorch's. Delete the then defunct code from torchchat

Alternatives

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Additional context

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RFC (Optional)

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