Delete loop unroll in SDPA op (#2438)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2438

Delete #pragma unroll as it's causing compiler errors when building llama/runner.cpp for AOSP targets

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Reviewed By: kimishpatel

Differential Revision: D54916248

fbshipit-source-id: 40b24e9d5f4af98bb961bacbeca94df8be71f989