Vanilla-fy DeepLabv3(+)

[DimitrisMantas]

I noticed that the DeepLab ASPP has a hardcoded 50% dropout layer at the exit. I'm not sure whether this is mentioned in the original papers, but I think it should be exposed to the user.

[qubvel]

Hi @DimitrisMantas! Thanks for the issue! I would appreciate this feature if you have the bandwidth to contribute 🤗