fix warpctc bug in dygraph mode; test=develop (#24119)

python/paddle/fluid/layers/loss.py

@@ -617,7 +617,7 @@ def warpctc(input,
     """
     helper = LayerHelper('warpctc', **locals())
     this_inputs = {'Logits': [input], 'Label': [label]}
-    if input_length and label_length:
+    if input_length is not None and label_length is not None:
         this_inputs['LogitsLength'] = [input_length]
         this_inputs['LabelLength'] = [label_length]