add additional stream priority for cuda streams (#101956)
Changes the StreamID encoding to use the last bit to distinguish between external and internal streams, 4 bits for IdType (DEFAULT, EXT or user-created streams possibly with high priority), and 5 bits for index. This allows us to have more stream priorities exposed to user (I'm currently setting 4, but that's easy to change now). Note, we are pre-creating all 32 streams in the pool per each allowed priority, I don't know if it's a problem in practice. Currently cuda 11.8/A100 GPUs allow 6 different stream priorities, the number may be different for the different cards/different cuda versions.
Previous callsites explicitly requesting high prioity stream (`isHighPriority=true`) are now getting the highest priority stream.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101956
Approved by: https://github.com/ezyang
