nvexec(continues_on): basic transition

[romintomasetti]

On the machines I could test, very basic transitions with nvexec::strean_context never work (it segfaults).

I already flagged this in:

• Transitioning from a nvexec::stream_context to another seg faults #1563

@ericniebler I haven't looked for a fix, but this bug seems quite important to fix. Feel free to work with this branch, I won't touch anymore 😉

For the record, here is the stack trace reported by cuda-gdb: output.log.

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[ericniebler]

thanks for the test cases. the stream scheduler has a curious history. it was written several years ago by a coworker who is a CUDA ninja, but was new to std::execution. i am not a CUDA guru (although i'm getting better). the stream scheduler code is ... challenging for me. when i try to fix bugs there i lose whole days. and sometimes my "fixes" cause other issues, or performance regressions.

all this is to say, if anybody has an interest in digging into this, they would have my undying gratitude. i agree the current situation is unsatisfactory.