Replace IR Node, Value, Output, NodePtr, NodeCast, etc
I did a lot of sedding and manual patchups. I wonder what the best
approach to these incremental refactors is. Renaming things to
explicitly use `torch::lazy::` namespace seems safest, and also because
it is a canonical fq namespace we can easily sed it later to clean up
once the whole codebase is landed. Using `using namespace..` is also
reasonable sometimes, but I got into some trouble using it and mostly
preferred the explicit approach.
