prevent "!switch state gets confused if model switching fails"

- If !switch were to fail on a particular model, then generate got
  confused and wouldn't try again until you switch to a different working
  model and back again.

- This commit fixes and closes #1547

ldm/generate.py

@@ -832,6 +832,7 @@ class Generate:
         model_data = cache.get_model(model_name)
         if model_data is None:  # restore previous
             model_data = cache.get_model(self.model_name)
+            model_name = self.model_name # addresses Issue #1547
 
         self.model = model_data['model']
         self.width = model_data['width']

ldm/invoke/model_cache.py

@@ -78,11 +78,12 @@ class ModelCache(object):
         else: # we're about to load a new model, so potentially offload the least recently used one
             try:
                 requested_model, width, height, hash = self._load_model(model_name)
-                self.models[model_name] = {}
-                self.models[model_name]['model'] = requested_model
-                self.models[model_name]['width'] = width
-                self.models[model_name]['height'] = height
-                self.models[model_name]['hash'] = hash
+                self.models[model_name] = {
+                    'model': requested_model,
+                    'width': width,
+                    'height': height,
+                    'hash': hash,
+                }
 
             except Exception as e:
                 print(f'** model {model_name} could not be loaded: {str(e)}')