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flashAttention was hardcoded to assume 4D [B, H, T, D] input, but models commonly pass 3D [B*H, T, D] after merging batch and head dims. With 3D input, qShape[3] was undefined, causing NaN to propagate through the entire forward pass. Now reads T and D from the last two dimensions regardless of rank.
fix: handle 3D input tensors in flashAttention
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