[Bug] Fix torch dynamo warning Dynamo detected a call to a functools.lru_cache

[yewentao256]

Purpose

(EngineCore_DP0 pid=2798273) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.
(EngineCore_DP0 pid=2798273)   torch._dynamo.utils.warn_once(msg)
(EngineCore_DP1 pid=2798274) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.
(EngineCore_DP1 pid=2798274)   torch._dynamo.utils.warn_once(msg)
(EngineCore_DP6 pid=2798279) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.
(EngineCore_DP6 pid=2798279)   torch._dynamo.utils.warn_once(msg)
(EngineCore_DP2 pid=2798275) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.
(EngineCore_DP2 pid=2798275)   torch._dynamo.utils.warn_once(msg)
(EngineCore_DP3 pid=2798276) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.
(EngineCore_DP3 pid=2798276)   torch._dynamo.utils.warn_once(msg)
(EngineCore_DP7 pid=2798280) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.
(EngineCore_DP7 pid=2798280)   torch._dynamo.utils.warn_once(msg)
(EngineCore_DP5 pid=2798278) /home/wentao/.venv/lib/python3.12/site-packages/torch/_dynamo/variables/functions.py:1692: UserWarning: Dynamo detected a call to a `functools.lru_cache`-wrapped function. Dynamo ignores the cache wrapper and directly traces the wrapped function. Silent incorrectness is only a *potential* risk, not something we have observed. Enable TORCH_LOGS="+dynamo" for a DEBUG stack trace.

The warning is from @cache in batch invariant variable, this PR fixes that

Test

All batch invariant unit tests pass

[gemini-code-assist]

Code Review

This pull request successfully resolves a torch.dynamo warning by replacing a functools.cache decorator with a module-level variable. While this fixes the warning, the changes to the test suite to accommodate this are not ideal. Specifically, the modification in conftest.py introduces a global side effect by setting an environment variable at the module level, which breaks test isolation. I've provided a critical suggestion to revert to a pytest fixture, using monkeypatch.setattr to ensure test isolation and proper setup/teardown. A related high-severity suggestion is made in test_batch_invariance.py to align with this improved testing pattern, making the test cleaner and more robust.

[ZJY0516]

cc @zou3519 and link #28867

[youkaichao]

thanks for the fix!

[zou3519]

Nice, thanks for fixing this.