Previously, we had a multi-phase strategy to identify models from their
files on disk:
1. Run each model config classes' `matches()` method on the files. It
checks if the model could possibly be an identified as the candidate
model type. This was intended to be a quick check. Break on the first
match.
2. If we have a match, run the config class's `parse()` method. It
derive some additional model config attrs from the model files. This was
intended to encapsulate heavier operations that may require loading the
model into memory.
3. Derive the common model config attrs, like name, description,
calculate the hash, etc. Some of these are also heavier operations.
This strategy has some issues:
- It is not clear how the pieces fit together. There is some
back-and-forth between different methods and the config base class. It
is hard to trace the flow of logic until you fully wrap your head around
the system and therefore difficult to add a model architecture to the
probe.
- The assumption that we could do quick, lightweight checks before
heavier checks is incorrect. We often _must_ load the model state dict
in the `matches()` method. So there is no practical perf benefit to
splitting up the responsibility of `matches()` and `parse()`.
- Sometimes we need to do the same checks in `matches()` and `parse()`.
In these cases, splitting the logic is has a negative perf impact
because we are doing the same work twice.
- As we introduce the concept of an "unknown" model config (i.e. a model
that we cannot identify, but still record in the db; see #8582), we will
_always_ run _all_ the checks for every model. Therefore we need not try
to defer heavier checks or resource-intensive ops like hashing. We are
going to do them anyways.
- There are situations where a model may match multiple configs. One
known case are SD pipeline models with merged LoRAs. In the old probe
API, we relied on the implicit order of checks to know that if a model
matched for pipeline _and_ LoRA, we prefer the pipeline match. But, in
the new API, we do not have this implicit ordering of checks. To resolve
this in a resilient way, we need to get all matches up front, then use
tie-breaker logic to figure out which should win (or add "differential
diagnosis" logic to the matchers).
- Field overrides weren't handled well by this strategy. They were only
applied at the very end, if a model matched successfully. This means we
cannot tell the system "Hey, this model is type X with base Y. Trust me
bro.". We cannot override the match logic. As we move towards letting
users correct mis-identified models (see #8582), this is a requirement.
We can simplify the process significantly and better support "unknown"
models.
Firstly, model config classes now have a single `from_model_on_disk()`
method that attempts to construct an instance of the class from the
model files. This replaces the `matches()` and `parse()` methods.
If we fail to create the config instance, a special exception is raised
that indicates why we think the files cannot be identified as the given
model config class.
Next, the flow for model identification is a bit simpler:
- Derive all the common fields up-front (name, desc, hash, etc).
- Merge in overrides.
- Call `from_model_on_disk()` for every config class, passing in the
fields. Overrides are handled in this method.
- Record the results for each config class and choose the best one.
The identification logic is a bit more verbose, with the special
exceptions and handling of overrides, but it is very clear what is
happening.
The one downside I can think of for this strategy is we do need to check
every model type, instead of stopping at the first match. It's a bit
less efficient. In practice, however, this isn't a hot code path, and
the improved clarity is worth far more than perf optimizations that the
end user will likely never notice.
