7951 anchors are not centered on grid cells

[hachoj]

This pull request resolves issue #7951 by correctly centering anchor boxes within their grid cells.

Description

Changes Made:

• Modified the anchor generation logic in monai/apps/detection/utils/anchor_utils.py to add a stride // 2 offset.
• Refactored the corresponding unit test in tests/apps/detection/utils/test_anchor_box.py to validate this new, correct behavior against the torchvision baseline by accounting for the offset.

Fixes #7951

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.
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