v0.6.2 release: updating and bug fixing

Features

• allow agent gc in gradio by @suluyana in #490
• update requirements.txt by @dahaipeng in #481
• allow choice if use llm in llama index by @suluyana in #483

Bug fixing

• fix none total token by @zzhangpurdue in #480
• fix model_server conflict by @dahaipeng in #485
• remove complicated import which might lead import error by @zzhangpurdue in #484
• Fix/rag no documents error by @suluyana in #486

Full Changelog: v0.6.1...v0.6.2