Phi4-mm: fix prompt processing, patch position ids and separator inserter (#2293)

This PR fixes issues related to patch position IDs and separator
inserter handling in the visual language models, and also updates some
test conditions for backend-specific behavior.

Adjust condition for setting chat_template based on model type.
Add a new parameter ATTENTION_BACKEND and introduce xfail conditions for
specific test cases.
Remove an unused member variable in the C++ VisionEncoderPhi4MM class.

---------

Co-authored-by: Alexander Suvorov <[email protected]>

Author: yatarkan