Trains the routed trunk's forward combination as a hard one-hot sample
(matching eval-time top-1 dispatch exactly) while keeping a smooth gradient
on the backward pass, targeting the train/eval mismatch identified as a
likely contributor to experts overlapping instead of partitioning in the
first energy-router rollout benchmark. Off by default (model.router.gumbel);
existing routed configs/checkpoints are unaffected.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
