Prettify the module names in LayerChoice

[ultmaster]

Currently, in many cases, LayerChoice induces long names like net.op_choice.choices.3.conv. There are two problems:

1. choices is basically useless. Users actually want to see net.op_choice.3.conv.
2. Use number to label the candidates in LayerChoice is not clear and easy to understand. Users should have the ability to give the modules customized names, like net.op_choice.sepconv3x3.conv. This is useful for debugging, visualizing and dumping search space for distributed search.

How to implement it

For point 1, I think adding the modules directly into _modules will work. We can refer to the implementation of nn.ModuleList.

For point 2, we can allow the op_choices in LayerChoice be more than a list (for example an OrderedDict). Refer to nn.Sequential.

As this involves changes of API, let's discuss it first.