Other than the dialect definitions (which will live in standard Dialect/
subdirectory), the goal here is to keep RefBackend-related code nested
in {include/npcomp,lib,test}/RefBackend.
This unblocks backend progress while the PyTorch frontend work is coming
online. Hopefully we can delete this soon.
See tools/mnist-playground/README.md for more context on what this tool
is for, next steps, and current status.