feat: add pyproject.toml for pip install and fix unit tests

[SophusDavid]

Summary

This PR introduces modern Python packaging support via pyproject.toml and fixes several unit test issues.

Changes

1. Modernize Installation (pyproject.toml)

• Add pyproject.toml with setuptools build system configuration
• Define core dependencies and optional extras for different PyTorch/inference engine combinations:
  • torch260-vllm, torch260-sglang
  • torch280-vllm, torch280-sglang
  • torch260-diffsynth, local-debug, gem, vision
• Update all documentation to reflect new installation method:
  • Replace pip install -r requirements_xxx.txt with pip install -e ".[extra]"
  • Update 20+ documentation files (English & Chinese)

2. Fix Unit Tests

• Fix test_frozen_lake.py test cases
• Fix test_traj_env_manager.py and debug variant
• Fix test_collator.py, test_cluster.py, test_generate_scheduler.py
• Fix FSDP2 CP equivalence tests
• Fix gradient norm unit tests
• Fix math dataset tests
• Fix Ray model forward tests
• Fix Megatron offload states tests

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