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-device.m

@@ -1184,6 +1184,7 @@ bool ggml_metal_device_supports_op(ggml_metal_device_t dev, const struct ggml_te
         case GGML_OP_RMS_NORM:
             return has_simdgroup_reduction && (ggml_is_contiguous_rows(op->src[0]));
         case GGML_OP_ROPE:
+        case GGML_OP_ROPE_BACK:
             return true;
         case GGML_OP_IM2COL:
             return ggml_is_contiguous(op->src[1]) && op->src[1]->type == GGML_TYPE_F32 && (op->type == GGML_TYPE_F16 || op->type == GGML_TYPE_F32);