Make OPT policy backward compatible with pre-OPT transformers versions (#2254)

deepspeed/module_inject/replace_policy.py

@@ -444,13 +444,12 @@ class HFOPTLayerPolicy(DSPolicy):
         try:
             import transformers
             HFOPTLayerPolicy._orig_layer_class = transformers.models.opt.modeling_opt.OPTDecoderLayer
+            if isinstance(DSPolicy.hf_model_config,
+                          transformers.models.opt.configuration_opt.OPTConfig):
+                self.pre_attn_norm = self.hf_model_config.do_layer_norm_before
         except:
             HFOPTLayerPolicy._orig_layer_class = None
 
-        if isinstance(DSPolicy.hf_model_config,
-                      transformers.models.opt.configuration_opt.OPTConfig):
-            self.pre_attn_norm = self.hf_model_config.do_layer_norm_before
-
     def get_hidden_heads(self):
         return self.client_module.self_attn.embed_dim, \
                 self.client_module.self_attn.num_heads