fix resnet50_variant pretrained bugs

ppcls/arch/backbone/variant_models/resnet_variant.py

 from paddle.nn import Conv2D
-from ppcls.arch.backbone.legendary_models.resnet import ResNet50
+from ppcls.arch.backbone.legendary_models.resnet import ResNet50, MODEL_URLS, _load_pretrained
 
 __all__ = ["ResNet50_last_stage_stride1"]
 
@@ -17,6 +17,7 @@ def ResNet50_last_stage_stride1(pretrained=False, use_ssld=False, **kwargs):
         return new_conv
 
     match_re = "conv2d_4[4|6]"
-    model = ResNet50(pretrained=pretrained, use_ssld=use_ssld, **kwargs)
+    model = ResNet50(pretrained=False, use_ssld=use_ssld, **kwargs)
     model.replace_sub(match_re, replace_function, True)
+    _load_pretrained(pretrained, model, MODEL_URLS["ResNet50"], use_ssld)
     return model