[Enhancement] Disable data race check by default, opt-in via env var (#2851)

The VerifyParallelLoop data race check can report false positives on
shared buffer stores whose per-thread addresses cannot be proven
distinct (e.g. indices loaded from fragments). Disable it by default;
users can opt in via the TILELANG_ENABLE_DATA_RACE_CHECK environment
variable or by explicitly setting the tl.disable_data_race_check pass
config to False.