Allow no label setting for HDF5DataLayer

[erogol]

As I try to define a autoencoder by using my custom data loaded in to HDF5, I see that we must define top: "label" in any way so it forces you to define a dummy set of label values for instances. Although, it does not cause any problem in optimization, it causes very disturbing set of console outputs as

Train net output #1: label=0
Train net output #2: label=0
Train net output #3: label=0
Train net output #4: label=0
... (goes to number of your data dimension)

I solved this by only inserting this to data_layers.hpp line 184

virtual inline bool AutoTopBlobs() const { return true; }

I don't know whether it is acceptable solution but in practice it is pretty fine. If it is not a convenient solution please make HDF5DataLayer able to accept no label configuration.

[ducha-aiki]

Use Silence layer.

[jeffdonahue]

The current dev version of HDF5DataLayer uses the HDF5 dataset names, any of which you may omit (including "label").