fix PP-OCRv4 bug for fp16 train (#2301)

paddlex/repo_apis/PaddleOCR_api/configs/PP-OCRv4_mobile_det.yaml

@@ -31,6 +31,7 @@ Architecture:
   Head:
     name: DBHead
     k: 50
+    fix_nan: True
 
 Loss:
   name: DBLoss

paddlex/repo_apis/PaddleOCR_api/configs/PP-OCRv4_server_det.yaml

@@ -31,6 +31,7 @@ Architecture:
     name: PFHeadLocal
     k: 50
     mode: "large"
+    fix_nan: True
 
 Loss:
   name: DBLoss