Created by: yanyq1990

如题，在对模型进行压缩时，train_optimizer参数是否需要设置，文档提到，train program如果带反向operators和优化更新相关的operators, train_optimizer需要设置为None，不是很明白这话的意思，是指的train program设置了优化器optimizer吗？参考示例代码的，哪怕是设置了optimizer，最后Compressor的参数train_optimizer也没有设置为None，并且如果在使用中设置为None，训练过程中的loss不下降，而设置train_optimizer不为None，会报错如下：

Traceback (most recent call last):
  File "pruning.py", line 119, in <module>
    compress()
  File "pruning.py", line 113, in compress
    com_pass.run()
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/contrib/slim/core/compressor.py", line 517, in run
    context.train_optimizer, context.place, context.scope)
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/contrib/slim/graph/graph_wrapper.py", line 417, in get_optimize_graph
    optimizer.minimize(target, no_grad_set=no_grad_var_names)
  File "</usr/local/lib/python2.7/dist-packages/decorator.pyc:decorator-gen-20>", line 2, in minimize
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/wrapped_decorator.py", line 25, in __impl__
    return wrapped_func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/dygraph/base.py", line 87, in __impl__
    return func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/optimizer.py", line 609, in minimize
    no_grad_set=no_grad_set)
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/optimizer.py", line 494, in backward
    no_grad_set, callbacks)
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/backward.py", line 706, in append_backward
    _append_backward_vars_(root_block, fwd_op_num, grad_to_var, grad_info_map)
  File "/usr/local/lib/python2.7/dist-packages/paddle/fluid/backward.py", line 518, in _append_backward_vars_
    op_desc.infer_shape(block.desc)
IndexError: _Map_base::at