fix vae dim error (#4650)

PaddleNLP/seq2seq/variational_seq2seq/model.py

@@ -230,11 +230,10 @@ class VAE(object):
         # `sample_output_layer` samples an id from the logits distribution instead of argmax(logits)
         # it will be used within BeamSearchDecoder
         sample_output_layer = lambda x: layers.unsqueeze(fluid.one_hot(
-            layers.unsqueeze(
             layers.sampling_id(
                 layers.softmax(
                     layers.squeeze(output_layer(x),[1])
-                ),dtype='int'), [1]),
+                ),dtype='int'),
                 depth=self.tar_vocab_size), [1])
 
         if mode == 'train':