[inductor] Add ir.Scan and lower aten.cumsum on CUDA (#106581)
This adds the `ir.Scan` node (currently only supported on CUDA) which re-uses the existing reduction kernel machinery to support different kinds of non-pointwise ops. Just like reductions it supports prologue and epilogue fusions and has both persistent and non-persistent kernel generation.
Currently this doesn't support the equivalent of `Reduction.create_multilayer` and will instead fall back to eager in those cases. This is because splitting into multiple kernel invocations ends up being far slower than cub's single kernel strategy which matches the performance of a copy kernel.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106581
Approved by: https://github.com/lezcano, https://github.com/atalman
