refactor(pdf_parse_union_core_v2): update import paths to use new package structure

Adapt import statements in `pdf_parse_union_core_v2.py` to reflect the updated packagestructure, changing from the `magic_pdf.v3.helpers` module to the `magic_pdf.model.v3`
module. This ensures compatibility with the revised directory layout.

magic_pdf/pdf_parse_union_core_v2.py

@@ -140,7 +140,7 @@ class ModelSingleton:
 
 
 def do_predict(boxes: List[List[int]], model) -> List[int]:
-    from magic_pdf.v3.helpers import prepare_inputs, boxes2inputs, parse_logits
+    from magic_pdf.model.v3 import prepare_inputs, boxes2inputs, parse_logits
     inputs = boxes2inputs(boxes)
     inputs = prepare_inputs(inputs, model)
     logits = model(**inputs).logits.cpu().squeeze(0)