[FEATURE] "Container" slimmable model

[sdatkinson]

Inspired by the example in #239, this Issue is to implement the "ContainerModel" class.

As far as a .nam file goes, I'll propose the following:

{
    "version": "0.6.1",
    "architecture": "SlimmableContainer",
    "config": {
        "submodels": [
            {
                "max_value": 0.5,
                "model": {...}
            },
            {
                "max_value": 1.0,
                "model": {...}
            }
        ]
    },
    "weights": [],
    "sample_rate": 48000
}

I'll attach a complete model that (I think) should work.

A few notes:

• weights is empty but is included for legacy reasons. (I kinda want to get away from that anyways in favor of a better laid-out format. Maybe we officially make it optional with this? We'll still require it for existing model types for now.)
• The constructor may want to do a variety of sanity checks, like asserting that the submodels have the same expected sample rate (though perhaps that can be relaxed in the future and things like sample rate-independent models (e.g. Carson et al., 2024) might be implemented as special cases of slimmable NAMs.

[jfsantos]

@sdatkinson can you please attach the example model? Working on this now.