[ROCm] Implement empty graph node support and safeguards for HIP command buffer

[phambinhfin]

📋 Summary of Changes

• Implement CreateEmptyNode for the ROCm command buffer backend using hipGraphAddEmptyNode, enabling empty graph nodes to serve as dependency synchronization points in HIP graphs.
• Fix SetPriority to silently succeed on HIP, since the HIP runtime does not expose a per-node priority API. This allows collectives using StreamPriority::Highest to be captured into HIP graphs without errors.
• Add empty traced graph detection in RocmCommandBuffer::Trace() to return a clear error when stream capture produces a graph with no operations, preventing opaque runtime crashes during graph instantiation.
• Implement PrepareFinalization to insert an empty node into empty body graphs before finalization, ensuring graph instantiation remains safe when conditional body graphs are empty.

🎯 Justification

When command buffers are enabled for collectives on ROCm, the thunk emitter creates EmptyCmd nodes for collective-done thunks (AllReduceDone, AllGatherDone, etc.) and WaitForStreams thunks that have control predecessors. These flow down to CreateEmptyNode, which was returning UnimplementedError on ROCm, causing crashes. Additionally, SetPriority was returning UnimplementedError, which also crashed when collectives with StreamPriority::Highest were captured into HIP graphs.

🚀 Kind of Contribution

🐛 Bug Fix, ✨ New Feature, ✏️ Tests

✏️ Unit Tests

Four platform-agnostic test cases added to gpu_command_buffer_test.cc:

• EmptyNodeWithKernel -- empty node as root with a dependent kernel launch
• EmptyNodeOnly -- command buffer with a single empty node (exercises PrepareFinalization)
• EmptyNodeChain -- chain of 3 empty nodes followed by a kernel (multi-hop dependency propagation)
• EmptyNodeAsDependencyBarrier -- kernel -> empty -> kernel ordering, validating the core use case for collective synchronization

✏️ Execution Tests

Built and ran //xla/stream_executor/gpu:gpu_command_buffer_test with --config=rocm on MI300X:

[==========] Running 4 tests from 1 test suite.
[ RUN      ] GpuCommandBufferTest.EmptyNodeOnly
[       OK ] GpuCommandBufferTest.EmptyNodeOnly (1314 ms)
[ RUN      ] GpuCommandBufferTest.EmptyNodeChain
[       OK ] GpuCommandBufferTest.EmptyNodeChain (15 ms)
[ RUN      ] GpuCommandBufferTest.EmptyNodeWithKernel
[       OK ] GpuCommandBufferTest.EmptyNodeWithKernel (10 ms)
[ RUN      ] GpuCommandBufferTest.EmptyNodeAsDependencyBarrier
[       OK ] GpuCommandBufferTest.EmptyNodeAsDependencyBarrier (9 ms)
[  PASSED  ] 4 tests.

Full suite: 8 passed, 0 failed, 35 skipped

[ezhulenev]

FYI I'm working on removing that part from command buffers as a part of #39382, and WaitForStreams will be deleted after that. Though of course let's merge this one, as empty commands might be useful later for some other use cases.

[phambinhfin]

@beckerhe @dimitar-asenov Could you also take a look so this can be merged.