Add an option to cache libtorch/ releases if you don't want to
download the latest. Add an option to enable source builds.
TESTS:
macOS: verify with / without cache downloads
verify source builds -- shared and static
Linux: Build Tests and Release builds
We recently noticed that it is necessary to update the shape lib while
doing an LLVM bump: https://github.com/llvm/torch-mlir/pull/973
We didn't have a place to document that this was a necessary step, so
add a basic section to hold that and the basic instructions for how to
do a bump.
powderluv
Sean Silva
Sambhav Jain