Replace index_select with gather to avoid randomness in controlling speed reduction

[ForFishes]

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• 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.

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Description

Replace index_select with gather to avoid randomness in controlling speed reduction

[paddle-bot]

Thanks for your contribution!