Add pretrain option for DenseNet and SENet

[yiheng-wang-nv]

So far, Class DenseNet and Senet do not have an option to load pretrained weights for specific arch. Users need to call a function, such as densenet121 and se_resnet50 to load pretrained weights. It may not be very convenient for some cases that only Class type are supported to call.
Let me submit a PR to improve it.

[yiheng-wang-nv]

Under default settings, given an arch should be enough to determine the network structure, but my last updates still need the user to input both pretrained_arch and other parameters to get the network. I'll submit a new PR to improve it again.