--load_best=true should force checkpoint to be loaded

[favyen2]

Two issues:

• Currently even if best.ckpt or other checkpoint exists in the project folder, with --load_best=true the checkpoint will only be loaded if last.ckpt also exists since the first check is for last.ckpt. I think it should be updated so with --load_best=true it can load any checkpoint. If this is updated, then we can update the docs/ to avoid needing to rename best.ckpt to last.ckpt when downloading checkpoints.
• It makes sense for --autoresume=true to continue even if there's no existing checkpoint, but I think --load_best=true should only continue if there's checkpoint instead of continuing with random initialization. Otherwise it is not obvious what the problem is when you are running model test or a prediction pipeline that it is really using random weights.