Fix sharding rank mismatch for LoRA weights and optimizer states in grpo_gemma.ipynb notebook

[rajasekharporeddy]

Issue:
Applying 3D PartitionSpec annotations (inherited from the base model's scanned layers) to 2D LoRA weights and their optimizer states causes a ValueError during GRPO training.

Fix:

• Added a dimensionality check (len(spec) <= x.ndim) in get_lora_model to safely fall back invalid specs to None for 2D LoRA weights.
• Applied the same check to the optimizer states by patching _shard_optimizer in the PeftTrainer.

Checklist

• I have verified that my change does not break existing code and all unit tests pass.
• My PR is based on the latest changes of the main branch (if unsure, rebase the code).
• I have signed the Contributor License Agreement.
• I have followed Contribution Guidelines.

[gemini-code-assist]

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