Add benchmark configuration and scripts for convnets.

[qingqing01]

Fix #6008

For the convenience of profiling, we need to add benchmark mode and running scripts.

• Add typical network ResNet for the benchmark.
• Use dataset Flowers-102 instead of CIAR. Since data shape of former is (3, 224, 224), and the latter is (3, 28, 28). The former data shape is more typical.
• After run python benchmark_convnet_model.py, some time info will be printed out.

The usage is：

optional arguments:
  -h, --help            show this help message and exit
  --model MODEL         The model architecture: (resnet).
  --batch_size BATCH_SIZE
                        The minibatch size.
  --iterations ITERATIONS
                        The number of minibatches.
  --pass_num PASS_NUM   The number of minibatches.
  --order ORDER         The data order: (NCHW, NHWC).
  --device DEVICE       The data order: (GPU, CPU).
  --infer_only          If set, run forward only.
  --use_cprof           If set, use cProfile.
  --use_nvprof          If set, use nvprof for CUDA.

[chengduoZH]

You can use "choices" in parser.add_argument.

[chengduoZH]

Writing this into 'help'? "Only support NCHW order now."

[chengduoZH]

place = fluid.CPUPlace() if fluid.CPUPlace() else fluid.GPUPlace(0)