Merge MoEFlexTokenLayer to MoELayer

[zhangyuqin1998]

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

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PR changes

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Description

Dsv3 dev merge flex token

[paddle-bot]

Thanks for your contribution!