[densenet] adding drop_rate in model params

The modified definition of the densenet model allows the user to pass a dropout rate in the model's constructor. The default dorp-rate is set to 0.1.

added New Feature label

requested review from @dcarron

assigned to @ojimenez

approved this merge request

enabled an automatic merge when the pipeline for 854a730e succeeds

merged

mentioned in commit f347ee8b