Make residual copy be GPU-friendly

[blegat]

Otherwise we get ERROR: Scalar indexing is disallowed. as it is slow

No GPU on so here is the result:

$ julia --project=test test/eval_residual_gpu.jl
Test Summary:                      | Pass  Total     Time
test_eval_residual_gpu_matches_cpu |    2      2  1m04.5s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.22%. Comparing base (2bec169) to head (62a4f0b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   92.23%   92.22%   -0.01%     
==========================================
  Files          25       25              
  Lines        3219     3217       -2     
==========================================
- Hits         2969     2967       -2     
  Misses        250      250              

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