feat: remove string fmt allocation in hot loop in BpeBuilder::build
feat: remove string fmt allocation in hot loop in BpeBuilder::build: a task in repo2rlenv-pr-diff (Harbor dataset). Title: feat: remove string fmt allocation in hot loop in BpeBuilder::build
The task
**Title:** feat: remove string fmt allocation in hot loop in `BpeBuilder::build`
Part of FineEnvs/repo2rlenv-pr-diff.