Specify weights or layer for dump weights callback

[szaman19]

Currently, the DumpWeights callback iterates through all the weights of a model and writes them to a specified directory.

Implementation here: https://github.com/LLNL/lbann/blob/9ca712cc3d38e010e54354a3013414c029c2aa85/src/callbacks/dump_weights.cpp#L285

For even moderately large models outputting all the weights is very time-consuming. It could be helpful for debugging purposes to have the ability to specify particular weights or layers in this callback to only dump those specific weights.

[benson31]

We should make DumpWeights, DumpGradients, and DumpErrorSignals just match the interface of DumpOutputs, which supports this feature.