add model_config support in TransformersModel

[Jonnathanz]

This pull request adds support for the model_config parameter in the TransformersModel class. With this change, it's now possible to pass a dictionary containing specific configuration options for model loading (via AutoModelForCausalLM.from_pretrained or AutoModelForImageTextToText.from_pretrained), separating these settings from the kwargs used in the generate() method.

Highlights:

Quantization support: Enables the use of configurations such as quantization_config (e.g., for 4-bit quantization using BitsAndBytes), as well as other parameters like torch_dtype and device_map.

Flexible model initialization: Users can now customize model loading with a wide range of parameters without interfering with generation-specific arguments.

Clear separation of concerns: Model configuration is handled through the model_config dictionary, while generation parameters remain in **kwargs during the generate() call.

This update improves customization options during model initialization, making the framework more versatile and suitable for models requiring specific loading configurations.

Open to feedback — happy to refine the implementation as needed.

Example:
```python
>>> from transformers import BitsAndBytesConfig
>>> from smolagents import CodeAgent, TransformersModel

>>> model_id = "Qwen/Qwen2.5-Coder-32B-Instruct"

>>> bnb_config = BitsAndBytesConfig(
...     load_in_4bit=True,
...     bnb_4bit_compute_dtype="float16",
...     bnb_4bit_use_double_quant=True,
...     bnb_4bit_quant_type="nf4"
... )

>>> model = TransformersModel(
...     model_id,
...     device_map="auto",
...     torch_dtype="auto",
...     trust_remote_code=True,
...     model_config={'quantization_config': bnb_config},
...     max_new_tokens=2000
... )

>>> agent = CodeAgent(tools=[], model=model)

>>> result = agent.run("Explain quantum mechanics in simple terms.")
>>> print(result)
"Quantum mechanics is a branch of physics that studies the behavior of particles at the smallest scales, such as atoms and subatomic particles. Unlike classical physics, which..."