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
• 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
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[Xuanwo]

Mostly LGTM. The only question to me is how can we test this PR?

[Xuanwo]

Thanks！