Replace the manual safetensors/dat file materialization in
_gather_state_dict with accelerate's get_state_dict_offloaded_model().
The old code only handled disk-offloaded weights but failed for the 398
CPU-offloaded meta tensors managed by accelerate's AlignDevicesHook.
Pre-move all GPU tensors to CPU before materialization to prevent CUDA
OOM when align_module_device restores non-hooked params to CUDA during
its __exit__ cleanup.
Update test to verify the safety net catches unmaterialized meta tensors
after the accelerate path (the old test checked for a missing offload
directory, which this codepath no longer uses).
Joseph Magly