torchrun-style process-per-GPU: launch_processes spawns one worker process per
GPU (re-exec current_exe with XTRAIN_{RANK,WORLD,LOCAL_RANK,NCCL_ID} env),
mints the ncclUniqueId once in the launcher and hex-injects it via env (no
shared FS/TCP, race-free). worker_env/run_worker read the env, bind the device
(own CUDA context), DdpContext::init + build_model + train_rank reused from T8
UNCHANGED. hex_encode/decode_unique_id are host-testable pure fns.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Gahow Wang
ffd548b80b