Created by: luotao1

• add ParallelDo example for multi-thread CPU training in benchmark/fluid. For four models in benchmark/fluid, only add ParallelDo example for mnist and resnet now.
• add --cpus arguments to specify the number of cpus. The usage likes:

python fluid_benchmark.py --model resnet --batch_size=64 --pass_num 5 --device CPU --data_set cifar10 --no_test --cpus=20

• remove unused batch_size_tensor = fluid.layers.create_tensor(dtype='int64'), since fluid.layers.accuracy(input=predict, label=label) will auto create it. https://github.com/PaddlePaddle/Paddle/blob/ee9832a7a9fc314995e529f715656ba4b3b0e2b3/python/paddle/fluid/layers/metric.py#L28-L39