Layers with heterogeneous data types

[hannes-brt]

It currently doesn't seem to be possible to define layers with bottom and top of different data types. I noticed @jeffdonahue's EmbedLayer (#1872) which works around this by casting floating point inputs to int. Specifically, I was hoping to implement a layer that encodes DNA sequence in 1-of-n encoding using Blob<char> as bottom and Blob<dtype> as top. This doesn't seem possible with the current architecture so I'll probably have to work around it by pre-encoding all my data. However, maybe it will be possible to make the architecture flexible enough to allow this sort of thing.