fix: model loading and deleting logic inside useLLM hook (#404)

## Description
Move try/catch inside async function, delete model only when preventLoad
is set to false.

### Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] Documentation update (improves or adds clarity to existing
documentation)

### Tested on

- [x] iOS
- [ ] Android

### Checklist

- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have updated the documentation accordingly
- [x] My changes generate no new warnings

Author: jakmro

packages/react-native-executorch/src/hooks/natural_language_processing/useLLM.ts

@@ -46,19 +46,19 @@ export const useLLM = ({
     setDownloadProgress(0);
     setError(null);
 
-    if (!preventLoad) {
+    if (preventLoad) return;
+
+    (async () => {
       try {
-        (async () => {
-          await model.load({
-            modelSource,
-            tokenizerSource,
-            tokenizerConfigSource,
-          });
-        })();
+        await model.load({
+          modelSource,
+          tokenizerSource,
+          tokenizerConfigSource,
+        });
       } catch (e) {
         setError(e);
       }
-    }
+    })();
 
     return () => {
       model.delete();