fix: use variable frame_num instead of hardcoded 81 in FLF2V mask#583

wishhyt wants to merge 1 commit intoWan-Video:mainWan-Video/Wan2.1:mainfrom

wishhyt:fix/flf2v-hardcoded-frame-numwishhyt/Wan2.1:fix/flf2v-hardcoded-frame-numCopy head branch name to clipboard