openvino: gate fallback with env var

[zhaixuejun1993]

This pull request introduces a new environment variable to the OpenVINO integration, allowing fallback behavior to be enabled or disabled. The main changes add support for this variable and update the logic for model splitting based on its value.

OpenVINO Fallback Support:

• Added GGML_OPENVINO_ENABLE_FALLBACK to the list of recognized environment variables in ggml_openvino_device_config::init(), enabling configuration of fallback behavior.
• Updated is_model_splitted() in utils.cpp to check the GGML_OPENVINO_ENABLE_FALLBACK environment variable and short-circuit the function if fallback is not enabled, simplifying control flow for model splitting logic.## Overview

Additional information

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure:

[ravi9]

LGTM