[NNC] fix SyncThreads insertion and reenable CudaSharedMem test (#44909)
Summary:
A previous fix for masking Cuda dimensions (https://github.com/pytorch/pytorch/issues/44733) changed the behaviour of inserting thread synchronization barriers in the Cuda CodeGen, causing the CudaSharedMemReduce_1 to be flaky and ultimately disabled.
The issue is working out where these barriers must be inserted - solving this optimally is very hard, and I think not possible without dependency analysis we don't have, so I've changed our logic to be quite pessimistic. We'll insert barriers before and after any blocks that have thread dimensions masked (even between blocks that have no data dependencies). This should be correct, but it's an area we could improve performance. To address this somewhat I've added a simplifier pass that removes obviously unnecessary syncThreads.
To avoid this test being flaky again, I've added a check against the generated code to ensure there is a syncThread in the right place.
Also fixed a couple of non-functional but clarity issues in the generated code: fixed the missing newline after Stores in the CudaPrinter, and prevented the PrioritizeLoad mutator from pulling out loads contained within simple Let statements (such as those produced by the Registerizer).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44909
Reviewed By: agolynski
Differential Revision: D23800565
Pulled By: nickgg
fbshipit-source-id: bddef1f40d8d461da965685f01d00b468d8a2c2f
