Add update_weights_from_disk endpoint to Engine

[jhinpan]

Motivation

Motivated by GRPO in TRL: trying to use update_weights_from_disk in offline engine mode for GRPO Training

Modifications

• Mainly add the update_weights_from_disk function in the engine.py just like update_weights_from_tensor and also update_weights_from_distributed
• Also add a unit test only for testing update_weights_from_disk in the offline engine mode

Checklist

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• Add unit tests as outlined in the Running Unit Tests.
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[zhaochenyang20]

merge test_engine_update_weights.py with test_update_weights.py. Like the style of test_update_weights_from_dist.py.

Also, whenever you add new tests, modify the run_suites.