Python net spec cleanups and top-less layers

[longjon]

This PR makes small stylistic improvements to the net spec code and includes an alternative implementation of the functionality provided by #2741.

While #2741 generates top-less layers, it breaks the meaningfulness of fn.tops (for fn a Function); in particular, it need no longer be true that fn.ntop == len(fn.tops), and iterating through fn.tops is no longer the same as iterating through fn's tops, which now requires a special case (https://github.com/BVLC/caffe/pull/2741/files#diff-57d5ca5bbea564c189da52386291f6d8R145), which, while mild in the current code, would need to propagate to future code that operates on the (Python-level) graph of Functions and Tops.

The fake top doesn't really need to be added to fn.tops, and could just be generated around https://github.com/BVLC/caffe/pull/2741/files#diff-57d5ca5bbea564c189da52386291f6d8R198, except that this would prohibit the top-less Function from discovering a name assigned to this fake top.

We could simply forgo naming of top-less layers, but a more complete solution is to allow naming of Functions (layers) separately from Tops (the fourth commit here), and have top-less layers return their Functions rather than any (fictitious) Tops (the fifth). It might be argued that this solution ungraciously overloads the semantics of net spec layer creation, since layers sometimes return Tops and sometimes Functions, but this is surely no worse than any solution in which top-less layers return something, since something cannot be a top that isn't. It's probably best to view this as an abbreviation of a syntax in which all layers return both their Function and all their Tops (an option which could actually be added in the future if it becomes desirable to create layers with distinct layer and first-top names).

A simple test for top-less layers is included.

Thanks @philkr for getting the ball rolling on toplessness, and let me know how you feel about this solution.

[jeffdonahue]

Cool! LGTM.

[shelhamer]

Sweet. Thanks Jon!