Fix DeepSpeed model preparation logic in Trainer class
Author: qgallouedec
Pull request: yes

The changes in #43711 caused the model to be never prepared when using DeepSpeed. When training you hit for example:


```
[rank0]: Traceback (most recent call last):
[rank0]:   File "/fsx/qgallouedec/trl/trl/scripts/grpo.py", line 193, in <module>
[rank0]:     main(script_args, training_args, model_args, dataset_args)
[rank0]:     ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank0]:   File "/fsx/qgallouedec/trl/trl/scripts/grpo.py", line 162, in main
[rank0]:     trainer.train()
[rank0]:     ~~~~~~~~~~~~~^^
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/trainer.py", line 2170, in train
[rank0]:     return inner_training_loop(
[rank0]:         args=args,
[rank0]:     ...<2 lines>...
[rank0]:         ignore_keys_for_eval=ignore_keys_for_eval,
[rank0]:     )
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/trainer.py", line 2537, in _inner_training_loop
[rank0]:     tr_loss_step = self.training_step(model, inputs, num_items_in_batch)
[rank0]:   File "/fsx/qgallouedec/trl/trl/trainer/grpo_trainer.py", line 1024, in training_step
[rank0]:     output = super().training_step(model, inputs, num_items_in_batch)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/trainer.py", line 3804, in training_step
[rank0]:     inputs = self._prepare_inputs(inputs)
[rank0]:   File "/fsx/qgallouedec/trl/trl/extras/profiling.py", line 202, in wrapper
[rank0]:     return func(self, *args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/trl/trl/trainer/grpo_trainer.py", line 1053, in _prepare_inputs
[rank0]:     generation_batch = self._generate_and_score_completions(generation_batch)
[rank0]:   File "/fsx/qgallouedec/trl/trl/trainer/grpo_trainer.py", line 1551, in _generate_and_score_completions
[rank0]:     ) = self._generate(prompts)
[rank0]:         ~~~~~~~~~~~~~~^^^^^^^^^
[rank0]:   File "/fsx/qgallouedec/trl/trl/trainer/grpo_trainer.py", line 1431, in _generate
[rank0]:     prompt_ids, completion_ids, logprobs, extra_fields = self._generate_single_turn(prompts)
[rank0]:                                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
[rank0]:   File "/fsx/qgallouedec/trl/trl/trainer/grpo_trainer.py", line 1237, in _generate_single_turn
[rank0]:     prompt_completion_ids = unwrapped_model.generate(
[rank0]:         **generate_inputs, generation_config=self.generation_config, disable_compile=True
[rank0]:     )
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/utils/_contextlib.py", line 124, in decorate_context
[rank0]:     return func(*args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/generation/utils.py", line 2638, in generate
[rank0]:     result = decoding_method(
[rank0]:         self,
[rank0]:     ...<5 lines>...
[rank0]:         **model_kwargs,
[rank0]:     )
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/generation/utils.py", line 2833, in _sample
[rank0]:     outputs = self._prefill(input_ids, generation_config, model_kwargs)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/generation/utils.py", line 3822, in _prefill
[rank0]:     return self(**model_inputs, return_dict=True)
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1776, in _wrapped_call_impl
[rank0]:     return self._call_impl(*args, **kwargs)
[rank0]:            ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1787, in _call_impl
[rank0]:     return forward_call(*args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/utils/generic.py", line 834, in wrapper
[rank0]:     output = func(self, *args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/models/qwen2/modeling_qwen2.py", line 475, in forward
[rank0]:     outputs: BaseModelOutputWithPast = self.model(
[rank0]:                                        ~~~~~~~~~~^
[rank0]:         input_ids=input_ids,
[rank0]:         ^^^^^^^^^^^^^^^^^^^^
[rank0]:     ...<6 lines>...
[rank0]:         **kwargs,
[rank0]:         ^^^^^^^^^
[rank0]:     )
[rank0]:     ^
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1776, in _wrapped_call_impl
[rank0]:     return self._call_impl(*args, **kwargs)
[rank0]:            ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1787, in _call_impl
[rank0]:     return forward_call(*args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/utils/generic.py", line 1001, in wrapper
[rank0]:     outputs = func(self, *args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/transformers/src/transformers/models/qwen2/modeling_qwen2.py", line 373, in forward
[rank0]:     inputs_embeds = self.embed_tokens(input_ids)
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1776, in _wrapped_call_impl
[rank0]:     return self._call_impl(*args, **kwargs)
[rank0]:            ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1882, in _call_impl
[rank0]:     return inner()
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/module.py", line 1830, in inner
[rank0]:     result = forward_call(*args, **kwargs)
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/modules/sparse.py", line 191, in forward
[rank0]:     return F.embedding(
[rank0]:            ~~~~~~~~~~~^
[rank0]:         input,
[rank0]:         ^^^^^^
[rank0]:     ...<5 lines>...
[rank0]:         self.sparse,
[rank0]:         ^^^^^^^^^^^^
[rank0]:     )
[rank0]:     ^
[rank0]:   File "/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.13/site-packages/torch/nn/functional.py", line 2567, in embedding
[rank0]:     return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)
[rank0]:            ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank0]: RuntimeError: Expected all tensors to be on the same device, but got index is on cuda:0, different from other tensors on cpu (when checking argument in method wrapper_CUDA__index_select)
```


this PR fixes it

Comments:
@ArthurZucker @Cyrilvallez ca we have this in the next patch release please (if any)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_43780). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

> cc @SunMarc do you know if our tests just did not catch or they are slow?

Slow tests but we definitely need to work on improving our tests. Coming soon !