T21 fixed --dropout under thread-per-GPU (train_ddp): added the flag, set
cfg.dropout, and made train_rank re-assert model.train() each step so the
training forward stays live across periodic eval flips. The process-per-GPU
launcher (train_ddp_mp) was left out: it never parsed --dropout, so cfg.dropout
stayed at Config::from_arch's 0.0 default, and the worker's model built with
dropout permanently disabled — silently, regardless of what the user passed.
The gap is the exact same launcher-wiring class the V9-PILOT caught: op-level
+ single-GPU tests pass, the DDP-thread T21 regression test passes, but the
proc-per-GPU launcher path was never exercised end-to-end with dropout>0.
Mirror bin/train_ddp exactly: parse --dropout (default 0, bit-identical
default), set cfg.dropout before build_model, print an ON banner on rank 0.
train_rank's per-step model.train() from T21 is reused unchanged (proc-per-GPU
uses the same train_rank).
Follow-up test that exercises this wiring end-to-end (GATE B loss-trace
divergence between p=0 and p=0.2 under process-per-GPU) lands in the next
commit.
Gahow Wang
86de6bfb51