How can we define a list of integers?

In many cases, we need pass a list integers to a program, such as a list containing the number of units per layer in a neural network.

For now, with `DEFINE_list`, we have to manually change the type of each element from string to int. And `DEFINE_multi_integer` may lead to a very long command line string since the argument name needs to appear multiple times.

Is it possible or easy to declare a default type of each element for `DEFINE_list`, like `argparse`?