Provide a busy indicator if we're processing a long prompt and make the

stop button work in the middle of processing a long prompt as well.

gptj.cpp

@@ -707,6 +707,9 @@ void GPTJ::prompt(const std::string &prompt, std::function<bool(const std::strin
             std::cerr << "GPT-J ERROR: Failed to process prompt\n";
             return;
         }
+        // We pass a null string to see if the user has asked us to stop...
+        if (!response(""))
+            return;
         ctx.n_past += batch.size();
         i = batch_end;
     }