Fix unittest, test=develop

python/paddle/fluid/dygraph/dygraph_to_static/partial_program.py

@@ -241,7 +241,7 @@ def _restore_out(self, out_vars):
         for i, idx in enumerate(self._outputs.var_ids):
             flatten_outputs[idx] = out_vars[i]
         outs = self._outputs.restore(flatten_outputs)
-        if len(outs) == 1:
+        if outs is not None and len(outs) == 1:
             outs = outs[0]
 
         return outs
@@ -260,14 +260,25 @@ def _remove_no_value(self, out_vars):
             if self._is_no_value(out_vars):
                 return None
             return out_vars
-        else:
-            res = tuple(var for var in out_vars if not self._is_no_value(var))
-            if len(res) == 0:
+        elif isinstance(out_vars, (tuple, list)):
+            if isinstance(out_vars, tuple):
+                res = tuple(
+                    var for var in out_vars if not self._is_no_value(var))
+            else:
+                # isinstance(out_vars, list)
+                res = [var for var in out_vars if not self._is_no_value(var)]
+
+            has_removed = (len(out_vars) > len(res))
+            # len(out_vars) > len(res) means we have removed var. This is
+            # preventing out_vars is empty or just one element at the beginning
+            if len(res) == 0 and has_removed:
                 return None
-            elif len(res) == 1:
+            elif len(res) == 1 and has_removed:
                 return res[0]
             return res
 
+        return out_vars
+
     def _set_grad_type(self, params):
         # NOTE: if user set sparse gradient mode, the param's gradient
         # will be SelectedRows, not LoDTensor. But tracer will just

python/paddle/fluid/dygraph/dygraph_to_static/program_translator.py

@@ -257,8 +257,9 @@ def from_func_spec(func_spec):
                 # 3. Builds program only once and returns the output Variables.
                 with param_guard(func_spec.parameters(False)):
                     outputs = static_func(*inputs)
-                if not isinstance(outputs, (tuple, list)):
-                    outputs = [outputs] if outputs else []
+                if not isinstance(outputs,
+                                  (tuple, list)) and outputs is not None:
+                    outputs = [outputs]
 
         return ConcreteProgram(
             inputs=inputs,

python/paddle/fluid/dygraph/dygraph_to_static/return_transformer.py

@@ -229,6 +229,8 @@ def visit_FunctionDef(self, node):
                     annotation=None,
                     type_comment=None)
             else:
+                # We need to initial return value as tuple because control flow
+                # requires input or output has same structure
                 return_value_nodes = gast.Tuple(
                     elts=[
                         gast.Name(