CUDA Graph Trace Compaction

CUDA graph replay gives each captured region its own stream, so a profiler trace opens in Perfetto as hundreds of near-empty rows. scripts/compact_cuda_graph_trace.py repacks the slices into a few semantic lanes (compute, NCCL all-gather, reduce-scatter, ..., memcpy, memset).

Visualization only. Slices keep their name, pid, ts, dur, and args -- only the display placement (tid/args.stream) changes. Hand the user the original for measurement and the compacted copy for reading.

The script is agent-maintained, not a torchtitan API -- edit it for the trace in front of you rather than working around it. Common tweaks: _classify_slice and GROUP_ORDER/GROUP_LABELS for new lane groupings, PP_ANNOTATION_PATTERN for a different annotation scheme. Keep the tests passing.