fix dynamic decode imperative (#28162)

Author: FrostML

python/paddle/fluid/layers/rnn.py

@@ -1395,7 +1395,7 @@ def _maybe_copy(state, new_state, step_mask):
         control_flow.increment(x=step_idx_tensor, value=1.0, in_place=True)
         step_idx += 1
 
-        control_flow.logical_not(nn.reduce_all(finished), cond)
+        cond = control_flow.logical_not(nn.reduce_all(finished))
         if max_step_num is not None and step_idx > max_step_num:
             break