fix mnist

[chengduoZH]

Error:

Traceback (most recent call last):
  File "fluid/mnist.py", line 167, in <module>
    run_benchmark(cnn_model, args)
  File "fluid/mnist.py", line 155, in run_benchmark
    test_avg_acc = eval_test()
  File "fluid/mnist.py", line 89, in eval_test
    accuracy.reset(exe)
NameError: global name 'accuracy' is not defined

[dzhwinter]

LGTM, but we need to simplify the inference API. It's too complicated.