Array API: subtract_overscan model fitting goes through numpy-only astropy.modeling

[mwcraig]

Problem

subtract_overscan with model=... fits via astropy.modeling:

ccdproc/ccdproc/core.py

Line 616 in 9d25eee

of = fitting.LinearLSQFitter()

astropy's fitters operate on numpy internally (LinearLSQFitter ends up in np.linalg.lstsq), so CuPy input hits the implicit-conversion error. This matches the test_subtract_overscan_model failures noted in #910.

Options

• Treat model fitting as a CPU-only operation under the policy in Array API: decide a policy for inherently CPU-only operations #935 (explicit host transfer + convert the evaluated model back to xp).
• For the common low-order polynomial case, an xp-native least-squares fit (xp.linalg.lstsq is in the array API linalg extension) could keep everything on-device.
• Longer term: array-API support in astropy.modeling (tracked in Tracking: upstream array-API blockers (astropy, reproject, astroscrappy) #940).

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Found during a review of the array API implementation from #885; follow-up to #909 / #910.