in the backward.cu, why the abs only added for .z and .w gradients, but not for .x and .y?

[charyang-ai]

in the backward.cu, why the abs only added for .z and .w gradients, but not for .x and .y?

`
// Update gradients w.r.t. 2D mean position of the Gaussian
atomicAdd(&dL_dmean2D[global_id].x, dL_dG * dG_ddelx * ddelx_dx);
atomicAdd(&dL_dmean2D[global_id].y, dL_dG * dG_ddely * ddely_dy);

// Homodirectional Gradient
atomicAdd(&dL_dmean2D[global_id].z, fabs(dL_dG * dG_ddelx * ddelx_dx));
atomicAdd(&dL_dmean2D[global_id].w, fabs(dL_dG * dG_ddely * ddely_dy));

// Update gradients w.r.t. 2D covariance (2x2 matrix, symmetric)
atomicAdd(&dL_dconic2D[global_id].x, -0.5f * gdx * d.x * dL_dG);
atomicAdd(&dL_dconic2D[global_id].y, -0.5f * gdx * d.y * dL_dG);
atomicAdd(&dL_dconic2D[global_id].w, -0.5f * gdy * d.y * dL_dG);
`