Persistent thread operator implementation

[jowens]

@neoblizz suggests we do a persistent-thread version of operators. CUDA has increasing support for persistent thread programming and cooperative-groups has good synchronization capabilities, and we can reasonably expect this support will improve in future hw/sw. The specific benefit is that if we use a PT model, we can achieve kernel fusion within a PT kernel between two operators.

@neoblizz notes that we should consider implementing alternate operators for our current operators that are implemented as PT operators at the device level. Programs would not need to change; instead a command-line switch could decide between a PT operator and the current non-PT operator.

[porumbes]

@YuxinxinChen take a look and discuss some of your current work with @neoblizz when you get a chance.