check if cmvn_file in config for u2

examples/librispeech/s2/conf/transformer.yaml

 # https://yaml.org/type/float.html
 # network architecture
 model:
+    cmvn_file:  
+    cmvn_file_type: "json"
     # encoder related
     encoder: transformer
     encoder_conf:

paddlespeech/s2t/models/u2/u2.py

@@ -860,7 +860,7 @@ class U2Model(U2DecodeModel):
             int, nn.Layer, nn.Layer, nn.Layer: vocab size, encoder, decoder, ctc
         """
         # cmvn
-        if configs['cmvn_file'] is not None:
+        if 'cmvn_file' in configs and configs['cmvn_file'] is not None:
             mean, istd = load_cmvn(configs['cmvn_file'],
                                    configs['cmvn_file_type'])
             global_cmvn = GlobalCMVN(