[fix] Autotuner: propagate the tuned function's return value

[aryaman-gupta]

Summary

Autotuner._run_with_hints calls self.fn(*args, **kwargs) on both branches (with and without compiler hints) but does not return it. As a result Autotuner.__call__ always yields None, even though _run_config does return self._run_with_hints(...) and __call__ returns _run_config(...) — the value is threaded all the way up to this one method, which drops it.

Any autotuned function that produces its result as a return value (rather than writing into a caller-owned output buffer) therefore silently gets None. Callers currently work around this by allocating an output tensor themselves, passing it in, and returning that buffer instead of trusting the autotuner's return.

Fix

Add the missing return on both branches of _run_with_hints.

Backward compatible:

• Callers that ignore the return value (in-place kernels writing into out) are unaffected — the fn still returns None, so the tuner still returns None.
• Callers whose fn returns its result now receive it.

Test

Adds two cases to tests/unit/test_autotune.py (GPU-free):

• test_call_returns_tuned_fn_value — the tuned fn's return value propagates through __call__ on both the search path and the cache-hit path. Fails without the fix (assert None == ('result', 64)).
• test_call_returns_none_when_fn_returns_none — an in-place fn that returns nothing still yields None, confirming the fix only forwards the value and doesn't fabricate one.

Run:

pytest tests/unit/test_autotune.py -k return -v