Add Mooncake DataProto transfer backend

[zxpdemonio]

Summary

This PR adds Mooncake as an optional ROLL transfer backend for structured DataProto transfer.

The implementation keeps ROLL-side integration aligned with the existing transfer backend boundary:

• ROLL keeps transfer_backend.put/get/delete and RemoteBatch semantics.
• Mooncake owns the structured object transfer implementation.
• Mooncake is integrated through the standard mooncake package APIs.
• The public transfer_backend.put(...) API remains unchanged.

What Changed

Mooncake Backend

• Added Mooncake backend support using mooncake.structured_object_store.MooncakeBundleTransfer.
• Stores ROLL DataProto-style payloads as Mooncake structured objects.
• Supports:
  • tensor batch fields
  • non_tensor_batch
  • mixed tensor/non-tensor payloads
  • lazy field materialization through ColumnRemoteBatch
  • cleanup through Mooncake DataProto handle cleanup
• Keeps Mooncake as an optional backend selected by config.

ROLL Compatibility

• Preserved the existing four-argument transfer_backend.put(partition, row_ids, fields, batch_size) API.
• Did not add Mooncake-specific kwargs to the shared transfer backend interface.
• Existing TransferQueue behavior remains unchanged.
• ROLL continues to own RemoteBatch / ColumnRemoteBatch semantics.

Node-scoped Client

• Keeps node-scoped Mooncake client support.
• This avoids initializing Mooncake resources independently in every worker process when node-level reuse is preferred.

Documentation

• Updated RemoteBatch transfer documentation.
• Documented Mooncake as an optional structured DataProto backend.
• Added Mooncake backend configuration example.
• Updated English and Chinese docs.

Tests

• Rewrote Mooncake transfer tests without fake/mock Mooncake modules.
• Added real RDMA-backed Mooncake round-trip coverage.
• Test uses standard mooncake command/package names and standard Mooncake environment variables.
• No hardcoded local repository paths are included in the test file.

Validation

PATH=/root/Mooncake-PR2050/build/mooncake-store/src:$PATH \
PYTHONPATH=/root/Mooncake-PR2050/mooncake-wheel:$PYTHONPATH \
MOONCAKE_MASTER=192.168.22.70:50051 \
MOONCAKE_LOCAL_HOSTNAME=192.168.22.70 \
MOONCAKE_PROTOCOL=rdma \
MOONCAKE_DEVICE_NAME=erdma_0 \
/root/roll/.venv/bin/python -m pytest -q \
  tests/distributed/scheduler/test_mooncake_transfer_backend.py

Result:

5 passed

Additional checks:

python -m py_compile
roll/distributed/scheduler/transfer_backend.py
tests/distributed/scheduler/test_mooncake_transfer_backend.py

git diff --check

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