[OpenVINO backend] supporting inference for Gemma, Mistral and GPT2 with ov backend

[Mohamed-Ashraf273]

Description of the change

As a part of my GSoC25 project to support inference with the openvino backend for Gemma , Mistral and GPT-2,
This is my PR for supporting Gemma , Mistral and GPT-2 pipelines.

import os
os.environ["KERAS_BACKEND"] = "openvino"
import keras_hub

model = keras_hub.models.GPT2CausalLM.from_preset(
    "gpt2_large_en", dtype="float16"
)
model.summary()
output = model.generate("Keras is ", max_length=20)
print("Generated text:", output)

Reference

https://docs.openvino.ai/2025/index.html
https://keras.io/api/
https://keras.io/keras_hub/

Colab Notebook

Checklist

• I have added all the necessary unit tests for my change.
• I have verified that my change does not break existing code and works with all backends (TensorFlow, JAX, and PyTorch).
• My PR is based on the latest changes of the main branch (if unsure, rebase the code).
• I have followed the Keras Hub Model contribution guidelines in making these changes.
• I have followed the Keras Hub API design guidelines in making these changes.
• I have signed the Contributor License Agreement.

[Mohamed-Ashraf273]

@fchollet
@mattdangerw
@rkazants
@divyashreepathihalli

[Mohamed-Ashraf273]

@mattdangerw
@divyashreepathihalli