refactor: remove stage use stream to avoid OOM.

[youngsofun]

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Summary

remove stage use stream instead of load all paths into one Vec to avoid OOM.

00_0004_copy_with_max_files.sh is changed too, no longer rely on the order of list of backends . cc @Xuanwo

Tests

• [ ] Unit Test
• [x] Logic Test
• [ ] Benchmark Test
• [ ] No Test - Explain why

Type of change

• [ ] Bug Fix (non-breaking change which fixes an issue)
• [ ] New Feature (non-breaking change which adds functionality)
• [ ] Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• [ ] Documentation Update
• [x] Refactoring
• [ ] Performance Improvement
• [ ] Other (please describe):

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This change is