fix(train): support eval-only mode (--num-rollout 0)#2109

EazyReal wants to merge 3 commits into

THUDM:mainTHUDM/slime:mainfrom

EazyReal:fix/eval-only-num-rollout-zeroEazyReal/slime:fix/eval-only-num-rollout-zeroCopy head branch name to clipboard