metal : implement rope_back operator (#24725)

Reuse existing rope kernels with a function constant to toggle forward/backward
rotation, avoiding duplicate kernel code.

Assisted-by: pi:llama.cpp/Qwen3.6-27B

ggml/src/ggml-metal/ggml-metal.metal

@@ -4358,6 +4358,7 @@ template [[host_name("kernel_mul_mv_bf16_bf16_short")]] kernel mul_mv_t_t_short_
 #endif
 
 constant bool FC_rope_is_imrope [[function_constant(FC_ROPE + 0)]];
+constant bool FC_rope_is_back   [[function_constant(FC_ROPE + 1)]];
 
 static float rope_yarn_ramp(const float low, const float high, const int i0) {
     const float y = (i0 / 2 - low) / max(0.001f, high - low);
@@ -4381,6 +4382,9 @@ static void rope_yarn(
     }
     *cos_theta = cos(theta) * mscale;
     *sin_theta = sin(theta) * mscale;
+    if (FC_rope_is_back) {
+        *sin_theta *= -1.0f;
+    }
 }
 
 // Apparently solving `n_rot = 2pi * x * base^((2 * max_pos_emb) / n_dims)` for x, we get