Add metadata.py and test_simple_storage_unit.py

[jianjunzhong]

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[Copilot]

Pull Request Overview

This PR adds core metadata structures for handling data batches and fields in the transfer queue system, along with terminology consistency improvements and comprehensive test coverage for storage functionality.

• Introduces metadata.py with data structures for managing field, sample, and batch metadata
• Updates variable naming from "per_tensor" to "per_field" for clarity and consistency across storage and controller components
• Adds extensive test coverage for the simple storage unit with various data types and client scenarios

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
verl/experimental/transfer_queue/metadata.py	New file containing metadata classes (FieldMeta, SampleMeta, BatchMeta, StorageMetaGroup) for structured data handling
verl/experimental/transfer_queue/storage.py	Updates variable names and comments from "per_tensor" to "per_field" terminology
verl/experimental/transfer_queue/controller.py	Updates method names and variable types to use "per_field" terminology consistently
verl/experimental/transfer_queue/client.py	Updates property access from deprecated fields to field_names
tests/experimental/transfer_queue/test_simple_storage_unit.py	New comprehensive test suite covering storage operations with various data types and client scenarios

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[Copilot]

This assumption should be documented more prominently, possibly as a class docstring note or method parameter documentation, as it's a critical constraint for the BatchMeta functionality.

[Copilot]

[nitpick] The tensor size 32768 appears as a magic number. Consider defining it as a constant (e.g., LARGE_TENSOR_SIZE = 32768) for better maintainability.