fix import error (#109)

* perf opt

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fix import error

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tools/llama/build_dataset.py

@@ -1,3 +1,4 @@
+import os
 import re
 from collections import defaultdict
 from multiprocessing import Pool