when delaying optimizer creation only prepare the model
Author: winglian
Pull request: yes
Labels: name: for patch

# 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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Comments:
cc @SunMarc 

cc @kmehant, if you can explain the change you tried to do, that would be helpful ! 

Hi @SunMarc thanks for looping me in! Appreciate it.

Ideally this block of code https://github.com/huggingface/transformers/blob/b31e9d19a6607aafdd921bc592897900712ba61d/src/transformers/trainer.py#L2344-L2350 should be doing model preparation using accelerate even for FSDP and TP (if you remember in the older version we TPlize the model in accelerate prepare which is not the latest cases so we are good) cases so that the model is wrapped and then the block does the optimizer creation taking in the accelerate prepared model parameters. After which it comes to the current block https://github.com/huggingface/transformers/blob/b31e9d19a6607aafdd921bc592897900712ba61d/src/transformers/trainer.py#L2353-L2362  which is being modified in the code where a prepare again for the model is not needed rather only for the optimizer since the previous created optimizer didn't undergo accelerate prepare. That was the rationale behind this change. Ideally instead of the change made in this PR, I think we should have simply modified 

```
self.model = self.accelerator.prepare(self.model)
```

to 
```
model = self.accelerator.prepare(self.model)
```
here - https://github.com/huggingface/transformers/blob/b31e9d19a6607aafdd921bc592897900712ba61d/src/transformers/trainer.py#L2349


OR

We can also go back to the older code too, since TPlizing the model is removed from accelerate prepare step which works as well.

```
model, self.optimizer = self.accelerator.prepare(self.model, self.optimizer)
```

for FSDP case. I can help in contribution if needed.


Nonetheless I +1 to @IlyasMoutawwakil to remove this all together since its always been a confusing parameter to me :)

cc: @ArthurZucker @winglian 

@SunMarc @IlyasMoutawwakil @ArthurZucker 

This is much more correct fix for this bug - PR: https://github.com/huggingface/transformers/pull/39177. The current PR breaks TP trainings (since prepare is not needed for TP and enforcing prepare would lead to DDP setting which fails). The aforementioned PR fixes for both FSDP and TP.