fix encode

[BossPi]

Before submitting

• Lint code. If there are lint issues, please format the code first.

# Install and register `pre-commit` in the project folder
pip install pre-commit && pre-commit install

# Process previous code files separately
pre-commit run --file XXXX.py

• Add test cases into tests folder. If there are codecov issues, please add tests cases first.

PR types

PR changes

Description

在encode处理messages时，处理连续出现的同一角色内容的情况，特别是assistant+function的情况

[paddle-bot]

Thanks for your contribution!