mergekit-mega: compound merging using multiple yaml documents in a single merge config

[nyxkrage]

This adds the a new script mergekit-mega that takes a yaml file with multiple merge configs specified as individual yaml documents and merges them in the proper order depending on interdependencies.

A couple of quirks that should probably be ironed out before merging

1. Currently there's no special handling for circular dependencies, and I'm unsure how it would break.
2. I dont know if the out_path in the yaml file is ideal, or if it should instead be a name, and the out_dir gets specified on the command line.
3. The huggingface repo regex probably doesn't cover all the edge cases in its current form
4. Unsure if just skipping over existing output directories should be the default behavior or if it should just abort, and have --force and --skip be 2 flags.

[cg123]

This is great! I'll look this over soon and give you some comments - think there are a few minor tweaks that would clean things up a bit. Excited to get this in.

You can ignore pre-commit complaining about the multi-document yaml file by the way - I'll change that setting.

[nyxkrage]

Added a basic check for circular dependencies and by henky's suggestion decided on using names for the individual merges and then specifying the working directory/out_path as a cli arg.
Using names also means that the regex for checking if its a local model or a huggingface repo is no longer needed

[cg123]

Looks pretty much good to me! I had one nitpick, and it looks like isort wants a change or two. Those aside I'm happy to merge this in.

When #67 makes its way in there's infrastructure to do exactly the kind of graph wrangling you wrote in here, which could make things cleaner - but that's a ways off.

Thanks for the PR!

[nyxkrage]

Alright, added some docstrings mostly to please pylint ran isort and moved the part about checking if its a dependent merge or not out to a separate function, so should be good to merge!

[nyxkrage]

Actually just realized that you can't use other models from the merge as a base model! 😅

[cg123]

Looks great! Thanks for putting this together.