[webgpu] Fix bug in 1D dispatch workgroups (microsoft#24519)

Fixed the bug in microsoft#24228 which causes the incorrect result for phi models
when flash attention is disabled.

Author: qjia7

onnxruntime/contrib_ops/webgpu/bert/attention.cc

@@ -155,7 +155,7 @@ Status AttentionProbsProgram::GenerateShaderCode(ShaderHelper& shader) const {
 
   shader.MainFunctionBody() << "if (m + local_id.y < uniforms.M && n + local_id.x < total_sequence_length) {\n"
                             << "  let headOffset = batch_head_idx * uniforms.M * uniforms.N;\n"
-                            << "  let outputIdx = headOffset + m + local_id.y * uniforms.N + n + local_id.x;\n"
+                            << "  let outputIdx = headOffset + (m + local_id.y) * uniforms.N + n + local_id.x;\n"
                             << "  var sum: f32 = " << (components_ == 4 ? "value.x + value.y + value.z + value.w" : (components_ == 2 ? "value.x + value.y" : "value")) << ";\n";
 
   shader.MainFunctionBody() << "  output[outputIdx] = output_value_t(sum * uniforms.alpha)";