New backbones and decoders/heads

[Joao-L-S-Almeida]

A set of backbone and decoder models must be supported in terratorch, as U-Nets and ASPP (Atrous Spatial Pyramid Pooling).

[Michal-Muszynski]

@romeokienzler we need to have a smoothly definable U-Net architecture with ASPP (Atrous Spatial Pyramid Pooling) in terratorch as our baseline for many diverse EO downstream tasks.

[romeokienzler]

@Michal-Muszynski closing as tested, please let us know if we need to reopen