Fix read-only numpy arrays in cross-validation (facebook#4794)

Summary:

I'm not sure why these are showing up as failures today, we migrated to Python scientific stack in july 2024, but these are deprecation warnings we should go ahead and take care of anyway. Figured would knock them out while they are causing me issues

Prompted after some failures in exports not related to my changes: https://github.com/facebook/Ax/actions/runs/21225324302/job/61070638075?fbclid=IwY2xjawPeXMFleHRuA2FlbQIxMQBicmlkETFRTkR6WlE4NHVrd3IyQXNlc3J0YwZhcHBfaWQBMAABHjTAiZi71n24w95hvzEewrKNPKOGzJisgR7t4qJ3APRMYlusgFC-gu7RLiSb_aem_Zk3pmTDonCFsJvZCTkpeMA


Add `.copy()` after `.numpy()` calls to ensure arrays are writeable.
PyTorch tensors converted via `.detach().cpu().numpy()` return read-only
arrays in some cases. The subsequent squeeze operations create read-only
views, and the in-place assignment `loo_covs[:, diag_idx, diag_idx] = loo_vars`
fails with "assignment destination is read-only" error.

Differential Revision: D91185467

Author: mgarrard

ax/adapter/cross_validation.py

@@ -240,12 +240,13 @@ def _efficient_loo_cross_validate(
         # This is a known limitation for fully Bayesian models and models with
         # non-Gaussian posteriors (such as PFNs).
         # Shape: n x 1 x m
-        loo_means = posterior.mixture_mean.detach().cpu().numpy()
-        loo_vars = posterior.mixture_variance.detach().cpu().numpy()
+        # Use .copy() to ensure arrays are writeable (numpy returns read-only views)
+        loo_means = posterior.mixture_mean.detach().cpu().numpy().copy()
+        loo_vars = posterior.mixture_variance.detach().cpu().numpy().copy()
     else:
         # Shape: n x 1 x m
-        loo_means = posterior.mean.detach().cpu().numpy()
-        loo_vars = posterior.variance.detach().cpu().numpy()
+        loo_means = posterior.mean.detach().cpu().numpy().copy()
+        loo_vars = posterior.variance.detach().cpu().numpy().copy()
 
     # Squeeze out the q dimension: n x 1 x m -> n x m
     loo_means = loo_means.squeeze(1)