Save the downloaded datasets to local directory

[lzjpaul]

In cnn and cifar_distributed_cnn examples, the downloaded files are in the /tmp directory, which may be lost after the machine is restarted.

[lzjpaul]

add an arguments for local directory and then save the downloaded files to this directory

[lzjpaul]

changed in example folder https://github.com/apache/singa/tree/dev/examples/cnn/data and https://github.com/apache/singa/tree/dev/examples/cifar_distributed_cnn/data

[lzjpaul]

resolved