Deprecate Cupy

[genedan]

Description

Sub-task for #601.

Is your feature request aligned with the scope of the package?

• Yes, absolutely!
• No, but it's still worth discussing.
• N/A (this request is not a codebase enhancement).

Describe the solution you'd like, or your current workaround.

No response

Do you have any additional supporting notes?

No response

[henrydingliu]

we shouldn't be taking functionality away before 1.0. my recommendation is a deprecation warning in 0.9.3.

[genedan]

Some places where we could issue the warning would be:

1. cl.options.set_option('ARRAY_BACKEND', 'cupy')
2. cl.options.set_option('ARRAY_PRIORITY', [...]) if the user puts either cupy or dask in front of numpy and sparse, or maybe we just issue a warning for setting cupy/dask as the first element, or maybe if the user bothers to change it at all?
3. Triangle.set_backend('cupy')

I'm not even sure if cupy works, I've had trouble installing the dependencies. I think maybe we should spend some, but not a huge amount of effort in not getting it to crash immediately so that we can issue a clean warning.

I think codecov may also issue some warnings about missing coverage in the cupy.py and dask.py modules, since we might be altering those files. If so, I think in this case it's OK to override and merge the warnings without adding extensive testing, other than maybe a small test for confirming that the warning works - since these two modules will be deprecated.

[priyam0k]

Happy to take this. I've implemented deprecation warnings only (no removal), per @henrydingliu's note — warnings on set_option('ARRAY_BACKEND','cupy'), 'cupy' in ARRAY_PRIORITY, and set_backend('cupy'), plus tests. PR incoming.