Attempting to remove defaultdict from the collation functions

[calebrob6]

Having Dataloaders return batches as defaultdicts with "image", "mask", etc. keys causes problems:

• In using ddp with lightning when the validation_step function returns a type defaultdict, TypeError: first argument must be callable or None occurs Lightning-AI/pytorch-lightning#10308 (comment)
• If a user (like me 🙂) had code that used the "bbox" key in older versions of torchgeo, but then upgrades to a newer version of torchgeo where we've replaced "bbox" with "bounds", then their code might silently fail.

Also, I don't see any particular benefit for our collation functions to returning defaultdicts vs. normal dicts.