Add TorchScript support for networks

[Nic-Ma]

Is your feature request related to a problem? Please describe.
Now we already fixed some TorchScript issues for several networks and added unit tests for them.
Still need to resolve TorchScript issues in other networks: SegResNet & SegResNetVAE, DenseNet, SENet, DynUNet.

Describe the solution you'd like
torch.jit.script() can work for these networks.

[Nic-Ma]

Hi @ericspod ,

I tried to fix these networks for TorchScript, but I didn't find the root cause immediately and I need to jump into another critical task.. Could you please help take look at this ticket as you are expert on TorchScript?
This ticket is very important for users to do additional development based on the trained model in MONAI.

Thanks in advance.