Add lr_crop_size in config

[joeyballentine]

I think crop size is a bit misleading for people that are new to training. Many times I've had to explain how training a 1x model with a crop size of 128 is equivalent to training a 4x model at 512. It seems silly to set the crop size to 32 for a 1x model, but it might seem less silly if every model by default used the same LR crop size of 32 regardless of scale.

At the very least I think a comment or something explaining this concept would suffice. This might be a case of us just needing better documentation rather than adding extra hand holding.

Thoughts?

[victorca25]

Use of "crop size" to refer to the target images crop size is used to match the definition of other existing projects for consistency. The comment in the documentation is a good idea, I'll add it on the "how to train" doc.