fix missing model._tp_size from ep refactor
Author: winglian
Pull request: yes
Labels: name: for patch

# What does this PR do?

#39501 refactored the TP into `distribute_model` and it seems like this part:

https://github.com/huggingface/transformers/pull/39501/files#diff-6b72b98c4c2dcfc6cc606843917733f5d858374fbc22a735ff483bbc0c1e63eaL5130-L5132
<img width="565" height="82" alt="Screenshot 2025-07-25 at 10 42 02 PM" src="https://github.com/user-attachments/assets/b3adcaed-e658-4d50-8561-cf4873d4fe59" />


should have been refactored into that new function, but now when doing TP, the model now doesn't have `._tp_size` set which is still needed so all TP training seems to be broken now. 

I've added it the logic back to the new `distribute_model` function to restore this functionality.

@ArthurZucker @S1ro1 

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Comments:
also added `._device_mesh` back too as TP training fails to save the final checkpoint:

```
[rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/transformers/trainer.py", line 3237, in _save_checkpoint
[rank0]:     self.save_model(output_dir, _internal_call=True)
[rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/transformers/trainer.py", line 3980, in save_model
[rank0]:     self._save(output_dir)   
[rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/transformers/trainer.py", line 4084, in _save
[rank0]:     self.model.save_pretrained(                                                                  
[rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/transformers/modeling_utils.py", line 3954, in save_pretrained
[rank0]:     state_dict = replace_state_dict_local_with_dtensor(state_dict, self._tp_plan, self._device_mesh)
[rank0]:                                                                                   ^^^^^^^^^^^^^^^^^
[rank0]:   File "/root/miniconda3/envs/py3.11/lib/python3.11/site-packages/torch/nn/modules/module.py", line 1940, in __getattr__
[rank0]:     raise AttributeError(                                                                        
[rank0]: AttributeError: 'LlamaForCausalLM' object has no attribute '_device_mesh'
```