Show loading model UI during model switch (#5691)

Summary:
Pull Request resolved: #5691

Destructing a model takes about 5 seconds of time. Previously, if the user switched models then "loading model..." would only show 5 seconds after the model was destructed.

To help UX we will show "loading model..." before we we destroy the model. This way the user will know that app has ack'd that a model refresh needs to happen.

Reviewed By: kirklandsign

Differential Revision: D62512275

fbshipit-source-id: 29a808ce1ded125752f846f3a7cd3e3d0e86854a

Author: cmodi-meta

examples/demo-apps/android/LlamaDemo/app/src/main/java/com/example/executorchllamademo/MainActivity.java

@@ -107,10 +107,6 @@ public void onStats(float tps) {
   }
 
   private void setLocalModel(String modelPath, String tokenizerPath, float temperature) {
-    if (mModule != null) {
-      mModule.resetNative();
-      mModule = null;
-    }
     Message modelLoadingMessage = new Message("Loading model...", false, MessageType.SYSTEM, 0);
     ETLogging.getInstance().log("Loading model " + modelPath + " with tokenizer " + tokenizerPath);
     runOnUiThread(
@@ -119,6 +115,12 @@ private void setLocalModel(String modelPath, String tokenizerPath, float tempera
           mMessageAdapter.add(modelLoadingMessage);
           mMessageAdapter.notifyDataSetChanged();
         });
+    if (mModule != null) {
+      ETLogging.getInstance().log("Start deallocating existing module instance");
+      mModule.resetNative();
+      mModule = null;
+      ETLogging.getInstance().log("Completed deallocating existing module instance");
+    }
     long runStartTime = System.currentTimeMillis();
     mModule =
         new LlamaModule(