Wrong default layers for Flux 2 Klein

### Describe the bug

Thanks a lot for taking the time to review this issue 🤗

There seems to be a mismatch between the default values exposed via CLI arguments and the layers that are actually used internally in the script:

Script:
[pipeline_flux2_klein.py](https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/flux2/pipeline_flux2_klein.py)
[train_dreambooth_lora_flux2_klein.py](https://github.com/huggingface/diffusers/blob/main/examples/dreambooth/train_dreambooth_lora_flux2_klein.py)

The argument definition specifies:

```
parser.add_argument(
"--text_encoder_out_layers",
type=int,
nargs="+",
default=[10, 20, 30],
help="Text encoder hidden layers to compute the final text embeddings.",
)
```
However, inside the implementation the following layers are actually used:
`hidden_states_layers: list[int] = (9, 18, 27)`
**Expected behavior**
The default CLI argument values should match the layers that are actually used internally, or the internal implementation should respect the provided CLI values.
**Actual behavior**
There is an off-by-one inconsistency between:
CLI defaults: [10, 20, 30]
Internal usage: (9, 18, 27)
This can lead to confusion and potentially incorrect assumptions when tuning or debugging training behavior.
**Possible cause**
This might be a copy-paste artifact from:
[train_dreambooth_lora_flux2.py](https://github.com/huggingface/diffusers/blob/main/examples/dreambooth/train_dreambooth_lora_flux2.py)
which uses a different setup (e.g. Mistral-based text encoder in a dev version), where layer indexing may differ.
**Additional context**
This discrepancy cost me ~72 GPU hours before I realized what was going on, so I figured it’s worth documenting 😅
**Suggested fix**
Either align defaults with (9, 18, 27)
Or make sure --text_encoder_out_layers is actually used consistently throughout the script

### Reproduction

No special setup required — this is directly visible from reading the script.

### Logs

```shell

```

### System Info

N/A

### Who can help?

_No response_

Commencez par src/diffusers/pipelines/flux2/pipeline_flux2_klein.py et examples/dreambooth/train_dreambooth_lora_flux2_klein.py, en comparant la valeur par défaut de la CLI pour --text_encoder_out_layers avec la valeur interne hidden_states_layers. Suivez le cheminement de l’argument jusqu’au calcul des embeddings de texte et vérifiez l’indexation prévue des layers. Le travail est terminé lorsque les valeurs par défaut et les layers réellement utilisés sont cohérents, et que les valeurs de CLI fournies explicitement sont respectées si tel est le comportement choisi.

Rédigé par le modèle d'indexation à partir du texte de l'issue.