[FEA]: Warn when the multiprocessing start method is "fork"

[Andy-Jost]

Is this a duplicate?

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Area

cuda.core

Is your feature request related to a problem? Please describe.

When using IPC capabilities in cuda.core, it is easy to make the mistake of starting new processes by the "fork" method, especially since that is the default for the multiprocessing module. When this occurs, child processes fail to construct Device objects, generating a CUDA_ERROR_NOT_INITIALIZED error. The solution is to use the "spawn" start method. Users may not be aware of this and the error message does not clearly indicate the problem.

Describe the solution you'd like

There are key points in cuda.core where it is clear that multiprocessing is being used. For instance, when calling a reduction (i.e., pickling) method that was registered with multiprocessing. In those cases, the library should perform a one-time check of the start method and warn if it is set incorrectly. Note that the start method can only be set once, so a one-time check while serializing is sufficient.

Describe alternatives you've considered

No response

Additional context

No response

[leofang]

This seems nice to have indeed. IIRC you added this warning and removed it before merging the IPC PRs. Is the plan to bring it back?