CachedMetadataFileSystemReader: shared cache

[sbak5]

What does this PR do ?

Class-level metadata cache keyed by checkpoint path so instances share metadata for the same directory. This will skip redundant loading of the same metadata file for Loading ShardedObject / ShardedTensors when fully-parallel loading is enabled.

⚠️ For major changes (either in lines of code or in its impact), please make sure to first share a design doc with the team. If you're unsure what's the best way to do so, contact the @mcore-oncall.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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• I have added relevant unit tests
• I have added relevant functional tests
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• I have added relevant documentation
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[copy-pr-bot]

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[dimapihtar]

LGTM. Thank you!

[ananthsub]

/ok to test 134a51e

[sbak5]

/ok to test e082b17

[sbak5]

@NVIDIA/core-adlr This PR is very straightforward.

[sbak5]

/ok to test 808a116

[sbak5]

/ok to test e7f4dda

[sbak5]

/ok to test 88c3256

[sbak5]

/ok to test 559fa55

[sbak5]

/ok to test 7044b18

[sbak5]

/ok to test ee17aec

[sbak5]

/ok to test 634cfbb

[sbak5]

/ok to test 2b9f1d9

[sbak5]

/ok to test ab2df52

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22601224832