Fix Bug of Used before Assignment (#34090)

* fix used before assign

* fix used before assign

python/paddle/fluid/tests/unittests/test_basic_lstm_api.py

@@ -68,13 +68,14 @@ def lstm_np(input,
 
         return new_hidden, new_cell
 
+    mask = None
+
     if batch_first:
         input = np.tranpose(input, [1, 0, 2])
         if mask is not None:
             mask = np.transpose(mask, [1, 0])
 
     batch_size = input.shape[1]
-    mask = None
     if sequence_length is not None:
         max_seq_len = input.shape[0]