Partial Loading PR 3.5: Fix pre-mature model drops from the RAM cache (#7522)

## Summary

This is an unplanned fix between PR3 and PR4 in the sequence of partial
loading (i.e. low-VRAM) PRs. This PR restores the 'Current Workaround'
documented in https://github.com/invoke-ai/InvokeAI/issues/7513. In
other words, to work around a flaw in the model cache API, this fix
allows models to be loaded into VRAM _even if_ they have been dropped
from the RAM cache.

This PR also adds an info log each time that this workaround is hit. In
a future PR (#7509), we will eliminate the places in the application
code that are capable of triggering this condition.

## Related Issues / Discussions

- #7492 
- #7494
- #7500 
- https://github.com/invoke-ai/InvokeAI/issues/7513

## QA Instructions

- Set RAM cache limit to a small value. E.g. `ram: 4`
- Run FLUX text-to-image with the full T5 encoder, which exceeds 4GB.
This will trigger the error condition.
- Before the fix, this test configuration would cause a `KeyError`.
After the fix, we should see an info-level log explaining that the
condition was hit, but that generation should continue successfully.

## Merge Plan

No special instructions.

## Checklist

- [x] _The PR has a short but descriptive title, suitable for a
changelog_
- [x] _Tests added / updated (if applicable)_
- [x] _Documentation added / updated (if applicable)_
- [ ] _Updated `What's New` copy (if doing a release after this PR)_

invokeai/backend/model_manager/load/load_base.py

@@ -57,20 +57,20 @@ class LoadedModelWithoutConfig:
         self._cache = cache
 
     def __enter__(self) -> AnyModel:
-        self._cache.lock(self._cache_record.key)
+        self._cache.lock(self._cache_record)
         return self.model
 
     def __exit__(self, *args: Any, **kwargs: Any) -> None:
-        self._cache.unlock(self._cache_record.key)
+        self._cache.unlock(self._cache_record)
 
     @contextmanager
     def model_on_device(self) -> Generator[Tuple[Optional[Dict[str, torch.Tensor]], AnyModel], None, None]:
         """Return a tuple consisting of the model's state dict (if it exists) and the locked model on execution device."""
-        self._cache.lock(self._cache_record.key)
+        self._cache.lock(self._cache_record)
         try:
             yield (self._cache_record.state_dict, self._cache_record.model)
         finally:
-            self._cache.unlock(self._cache_record.key)
+            self._cache.unlock(self._cache_record)
 
     @property
     def model(self) -> AnyModel:

invokeai/backend/model_manager/load/model_cache/model_cache.py

@@ -194,9 +194,15 @@ class ModelCache:
 
         return cache_entry
 
-    def lock(self, key: str) -> None:
+    def lock(self, cache_entry: CacheRecord) -> None:
         """Lock a model for use and move it into VRAM."""
-        cache_entry = self._cached_models[key]
+        if cache_entry.key not in self._cached_models:
+            self._logger.info(
+                f"Locking model cache entry {cache_entry.key} ({cache_entry.model.__class__.__name__}), but it has "
+                "already been dropped from the RAM cache. This is a sign that the model loading order is non-optimal "
+                "in the invocation code (See https://github.com/invoke-ai/InvokeAI/issues/7513)."
+            )
+        # cache_entry = self._cached_models[key]
         cache_entry.lock()
 
         try:
@@ -214,9 +220,15 @@ class ModelCache:
             cache_entry.unlock()
             raise
 
-    def unlock(self, key: str) -> None:
+    def unlock(self, cache_entry: CacheRecord) -> None:
         """Unlock a model."""
-        cache_entry = self._cached_models[key]
+        if cache_entry.key not in self._cached_models:
+            self._logger.info(
+                f"Unlocking model cache entry {cache_entry.key} ({cache_entry.model.__class__.__name__}), but it has "
+                "already been dropped from the RAM cache. This is a sign that the model loading order is non-optimal "
+                "in the invocation code (See https://github.com/invoke-ai/InvokeAI/issues/7513)."
+            )
+        # cache_entry = self._cached_models[key]
         cache_entry.unlock()
         if not self._lazy_offloading:
             self._offload_unlocked_models(0)