Fix read-only numpy arrays in cross-validation

[mgarrard]

Summary:
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

[meta-codesync]

@mgarrard has exported this pull request. If you are a Meta employee, you can view the originating Diff in D91185467.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.70%. Comparing base (6045c31) to head (ae9819d).

Files with missing lines	Patch %	Lines
ax/adapter/cross_validation.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4794   +/-   ##
=======================================
  Coverage   96.70%   96.70%           
=======================================
  Files         587      587           
  Lines       61294    61294           
=======================================
+ Hits        59273    59275    +2     
+ Misses       2021     2019    -2     

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