Update on "[Executorch] Add non-flash SDPA for decode"

Add cpu_sdpa template function in op_sdpa_impl.h that provides a
simpler SDPA implementation using standard GEMM (no tiling). This is
useful as a baseline and for cases where flash attention is not optimal.

The implementation uses a single SeqDim parameter for all tensors and
supports causal masking, attention masks, GQA, and multi-threading.

During decode (seq_len == 1), the tiled flash attention implementation
has unnecessary overhead from its blocking/tiling logic. The simpler
unfused SDPA path using direct GEMM is more efficient for single-query
attention, yielding ~25-30% decode throughput improvement on S25
(41 -> 53 tok/s for 1.4B parameter model).

This makes cpu_sdpa always available (previously gated behind
ET_USE_UNFUSED_SDPA) and dispatches to it when seq_len == 1 and
inputs are not quantized. Prefill continues to use flash attention.

Differential Revision: [D96044318](https://our.internmc.facebook.com/intern/diff/D96044318/)

[ghstack-poisoned]

Author: kimishpatel