fix update weights from disk in FSDP engine

[garrett4wade]

1. Skip initializing weight update groups when using disk-based update.
2. Write name_resolve entry first then wait for results.

NOTE: The same fix for the megatron engine will be applied in #324.

[gemini-code-assist]

Summary of Changes

Hello @garrett4wade, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request implements two key fixes for the FSDP engine's functionality when updating model weights from disk. It refines the sequence of operations during disk-based updates to ensure proper state management and avoids redundant or incorrect initialization steps, thereby improving the robustness and efficiency of the weight update process.

Highlights

• Disk-based Weight Update Ordering: Reordered the execution flow in _update_weights_from_disk to ensure the name_resolve entry is written before waiting for the future result, addressing a potential timing issue and ensuring proper operational sequence.
• Disk-based Weight Update Initialization: Introduced a conditional check in connect_engine to skip the initialization of distributed weight update groups when the update type is 'disk', preventing unnecessary setup for disk-based updates.

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[gemini-code-assist]

Code Review

This pull request introduces two fixes for updating weights from disk in the FSDP engine. First, it correctly reorders operations to prevent a potential deadlock by adding the weight update entry to name_resolve before waiting for the rollout engine's confirmation. Second, it skips the initialization of the distributed weight update group when using a disk-based update. My review identifies a potential issue with the implementation of the second fix. The use of an early return statement causes a final synchronization barrier to be skipped, which could lead to race conditions. I've suggested a refactoring to maintain this important synchronization point.

[nuzant]

LGTM.