Multi-LoRA SFT support FSDP2

[kevssim]

PR type

• Bug Fix
• New Feature
• Document Updates
• More Models or Datasets Support

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[gemini-code-assist]

Code Review

This pull request implements FSDP2 support for MultiLoraTransformersModel by integrating it into the shared strategy and lazy-wrap lifecycle and introducing sharding-aware parameter access helpers. Review feedback identifies critical bugs in the distributed tensor handling: _write_param_tensor may incorrectly double-shard local data, set_state_dict risks shape mismatches when applying global state to local shards, and get_state_dict returns sharded tensors that could lead to corrupt checkpoints. Furthermore, the model's initialization should be refactored to properly use the parent class, and internal imports should be moved to the module level.