Repository: huggingface/transformers
Pull request: when delaying optimizer creation only prepare the model

# What does this PR do?
Axolotl's CI caught a regression when we tried to upgrade to latest transformers. https://github.com/axolotl-ai-cloud/axolotl/actions/runs/15962262932/job/45016550543

PR #36132 introduced a regression breaking FSDP w llama
```
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/transformers/models/llama/modeling_llama.py", line 408, in forward                                                                                                          
stderr: [rank0]:     inputs_embeds = self.embed_tokens(input_ids)                                                                                                                                                                                              
stderr: [rank0]:                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                              
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1751, in _wrapped_call_impl                                                                                                               
stderr: [rank0]:     return self._call_impl(*args, **kwargs)                                                                                                                                                                                                   
stderr: [rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                                   
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1762, in _call_impl                                                                                                                       
stderr: [rank0]:     return forward_call(*args, **kwargs)                                                                                                                                                                                                      
stderr: [rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                                      
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/sparse.py", line 190, in forward                                                                                                                           
stderr: [rank0]:     return F.embedding(                                                                                                                                                                                                                       
stderr: [rank0]:            ^^^^^^^^^^^^                                                                                                                                                                                                                       stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/functional.py", line 2551, in embedding                                                                                                                            
stderr: [rank0]:     return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)                                                                                                                                                            stderr: [rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                            
stderr: [rank0]: RuntimeError: Output 0 of ViewBackward0 is a view and its base or another view of its base has been modified inplace. This view is the output of a function that returns multiple views. Such functions do not allow the output views to be modified inplace. You should replace the inplace operation by an out-of-place one.  
```
and FSDP+DPO+qwen

```
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/transformers/models/llama/modeling_llama.py", line 408, in forward                                                                                                          
stderr: [rank0]:     inputs_embeds = self.embed_tokens(input_ids)                                                                                                                                                                                              
stderr: [rank0]:                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                              
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1751, in _wrapped_call_impl                                                                                                               
stderr: [rank0]:     return self._call_impl(*args, **kwargs)                                                                                                                                                                                                   
stderr: [rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                                   
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1762, in _call_impl                                                                                                                       
stderr: [rank0]:     return forward_call(*args, **kwargs)                                                                                                                                                                                                      
stderr: [rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                                                      
stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/sparse.py", line 190, in forward                                                                                                                           
stderr: [rank0]:     return F.embedding(                                                                                                                                                                                                                       
stderr: [rank0]:            ^^^^^^^^^^^^                                                                                                                                                                                                                       stderr: [rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/functional.py", line 2551, in embedding                                                                                                                            
stderr: [rank0]:     return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)                                                                                                                                                            stderr: [rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                            
stderr: [rank0]: RuntimeError: Output 0 of ViewBackward0 is a view and its base or another view of its base has been modified inplace. This view is the output of a function that returns multiple views. Such functions do not allow the output views to be modified inplace. You should replace the inplace operation by an out-of-place one.  
```

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Changed files:
filename: src/transformers/trainer.py
patch: @@ -2357,7 +2357,7 @@ def _inner_training_loop(
                     model = self.accelerator.prepare(self.model)
                 else:
                     if delay_optimizer_creation:
-                        self.optimizer = self.accelerator.prepare(self.optimizer)
+                        model = self.accelerator.prepare(self.model)
                     else:
                         model, self.optimizer = self.accelerator.prepare(self.model, self.optimizer)
             else: