[FEA]: Add support for CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY in LaunchConfig

[lijinf2]

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Area

cuda.core

Is your feature request related to a problem? Please describe.

LaunchConfig does not expose CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY, which overrides the CPU wait policy (SPIN / YIELD / BLOCKING_SYNC) for a launch.

Parent tracking: #496.

Describe the solution you'd like

• Users can set CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY via LaunchConfig.
• Mapping to the native launch attribute is tested.

Describe alternatives you've considered

1. Use cuda.bindings.driver (CUlaunchAttribute / CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY) and cuLaunchKernelEx directly.
2. Set the stream attribute via cuStreamSetAttribute(..., CU_STREAM_ATTRIBUTE_SYNCHRONIZATION_POLICY, ...) instead of a per-launch attribute.

Downside of (1)/(2): leaves cuda.core users without a first-class LaunchConfig API; they must drop to low-level bindings and lose consistency with other LaunchConfig launch attributes (is_cooperative, programmatic_stream_serialization, etc.).

Additional context

Incremental LaunchConfig attribute coverage under #496 (same approach as #1334 for programmatic_stream_serialization).

Example (C API) — agent-generated; reviewed but not executed:

// Per-launch SPIN policy (lowest-latency host sync)
CUlaunchAttribute syncAttr = {
    .id    = CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY,
    .value = { .syncPolicy = CU_SYNC_POLICY_SPIN }
};

CUlaunchConfig config = {
    .gridDimX = 128, .gridDimY = 1, .gridDimZ = 1,
    .blockDimX = 256, .blockDimY = 1, .blockDimZ = 1,
    .sharedMemBytes = 0,
    .hStream = latencyCriticalStream,
    .attrs = &syncAttr,
    .numAttrs = 1
};

cuLaunchKernelEx(&config, kernel, NULL, NULL);

// Subsequent sync on this stream busy-waits on CPU
cuStreamSynchronize(latencyCriticalStream);