Fix: Apply `get_placeholder_mask` in Ovis2 (#40280)
* Refactor special image mask
* Refactor get_placeholder_mask method
* Revert "Refactor special image mask"
This reverts commit 9eb1828ae930329656d6f323a510c5e6033e1f85.
* Fix
* Revert "Refactor get_placeholder_mask method"
This reverts commit 07aad6484bb08d6351d5b605e9db574d28edcd15.
