fix: print proper unit "MB" when file downloading for LlamaCPP backend (#19778)

print proper unit "MB" when file downloading

Author: mshr-h

llama-index-integrations/llms/llama-index-llms-llama-cpp/llama_index/llms/llama_cpp/base.py

@@ -242,6 +242,7 @@ def _download_url(self, model_url: str, model_path: str) -> None:
                     for chunk in tqdm(
                         r.iter_content(chunk_size=chunk_size),
                         total=int(total_size / chunk_size),
+                        unit="MB",
                     ):
                         file.write(chunk)
             completed = True