Remove rebuild argument at the CLI level

[jeremyfowers]

@danielholanda please consider the following:

Back in the olden days, the rebuild argument was there to help manage the cache for users of the build API. We now have no one using the build API, so that has stopped being relevant.

In the TKML 1.0 era, the rebuild argument was primarily useful for "build once, benchmark elsewhere" workflows. However, we now have the turnkey cache benchmark command which is a much better way to benchmark a prebuilt model. rebuild has also stopped being relevant there.

In the TKML Refresh, commands are much more explicit. If I call turnkey -i model.py discover export-pytorch I expect it to do the thing! Not say "there was a cache hit, so I ignored your command." I find myself always using rebuild=always when I use TKML 2.0.

Finally, in TKML 2.0, it is easy to add a new front-end Stage along the lines of load-build, that would load a build from the cache and pass it to the next Stage. This would enable things like turnkey -i cache/* load-build benchmark to be a much more general replacement for turnkey cache benchmark

@danielholanda questions for you:

1. Can we get rid of the rebuild argument at the CLI level? My proposal is to set rebuild=always as the default at the CLI->build_api interface. We can keep rebuild=if_needed as the default in the build API.
2. Should we implement load-build and use it to phase out turnkey cache benchmark?

[jeremyfowers]

cc @danielholanda

[danielholanda]

yes, and yes. It is quite a change but load-build makes it worth it.