avoid 'invalid escape sequence' warning (#4832)

ppdet/modeling/ops.py

@@ -473,7 +473,7 @@ def distribute_fpn_proposals(fpn_rois,
                              pixel_offset=False,
                              rois_num=None,
                              name=None):
-    """
+    r"""
     
     **This op only takes LoDTensor as input.** In Feature Pyramid Networks 
     (FPN) models, it is needed to distribute all proposals into different FPN 
@@ -1274,7 +1274,7 @@ def box_coder(prior_box,
               box_normalized=True,
               axis=0,
               name=None):
-    """
+    r"""
     **Box Coder Layer**
     Encode/Decode the target bounding box with the priorbox information.