metal : fix graph capture and handle empty graphs (llama/29390)
- return early when the graph has no nodes
- drop the redundant reset of capture_compute: the decrement at the top
of the function already transitions the counter from 0 to -1, so a
capture happens exactly once
- hint at METAL_CAPTURE_ENABLED=1 in the capture error message
- pass capture_compute == 0 (not the raw counter) as use_capture to
ggml_metal_op_init, so GPU debug-group markers are only emitted on the
captured compute
Assisted-by: pi:llama.cpp/Qwen3.8-27B
whisper.cpp
4550b9db - metal : fix graph capture and handle empty graphs (llama/29390)
