move softmax loss GPU todo comment to header file

There's no need to have GPU stubs just for a comment, and the absence of
a GPU-specific implementation is made more obvious by the nonexistence
of the .cu file.
Merge pull request #1655 from longjon/softmax-loss-todo

Move softmax loss GPU todo comment from .cu file to header file
remove SoftmaxLossLayer CPU_ONLY stubs, since there is no GPU version
Merge pull request #1656 from longjon/fix-softmax-loss-stubs

Fix an error made in removing the SoftmaxLossLayer GPU stubs
add missing value support to SoftmaxLossLayer
add spatial normalization option to SoftmaxLossLayer

With missing values (and batches of varying spatial dimension),
normalizing each batch across instances can inappropriately give
different instances different weights, so we give the option of simply
normalizing by the batch size instead.