[Dy2Stat]Enhance nonlocal mechanism while nonlocal vars is empty

[Aurelius84]

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Bug fixes

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Describe

[Dy2Stat] Enhance nonlocal machanism while nonlocal vars is empty.

修复了当if-else的body函数中nonlocal names为空时，AST语法解析错误的问题，原始报错信息如下：

  File "/workspace/env3.7/lib/python3.7/site-packages/paddle/fluid/dygraph/dygraph_to_static/ifelse_transformer.py", line 60, in transform
    self.visit(self.root)
  File "/usr/lib/python3.7/ast.py", line 271, in visit
    return visitor(node)
  File "/usr/lib/python3.7/ast.py", line 326, in generic_visit
    value = self.visit(value)
  File "/usr/lib/python3.7/ast.py", line 271, in visit
    return visitor(node)
  File "/usr/lib/python3.7/ast.py", line 326, in generic_visit
    value = self.visit(value)
  File "/usr/lib/python3.7/ast.py", line 271, in visit
    return visitor(node)
  File "/workspace/env3.7/lib/python3.7/site-packages/paddle/fluid/dygraph/dygraph_to_static/ifelse_transformer.py", line 65, in visit_If
    node, self.root)
  File "/workspace/env3.7/lib/python3.7/site-packages/paddle/fluid/dygraph/dygraph_to_static/ifelse_transformer.py", line 527, in transform_if_else
    import pdb;pdb.set_trace()
  File "/workspace/env3.7/lib/python3.7/site-packages/paddle/fluid/dygraph/dygraph_to_static/variable_trans_func.py", line 85, in create_nonlocal_stmt_node
    return gast.parse(func_code).body[0]
  File "/workspace/env3.7/lib/python3.7/site-packages/paddle/utils/gast/gast.py", line 613, in parse
    return ast_to_gast(_ast.parse(*args, **kwargs))
  File "/usr/lib/python3.7/ast.py", line 35, in parse
    return compile(source, filename, mode, PyCF_ONLY_AST)
  File "<unknown>", line 1
    nonlocal
            ^
SyntaxError: invalid syntax

[0x45f]

LGTM