Fix: exps_dir

[gaozhihan]

1. Change the dir for saving experiment results from PROJECT_ROOT/experiments/ to separate dir for each dataset. E.g., PROJECT_ROOT/scripts/cuboid_transformer/sevir/experiments/ is for saving experiments on SEVIR dataset.
2. Delete the usage of cfg from from earthformer.config import cfg in all training scripts, except for setting the pretrained_checkpoints_dir at very beginning, like in https://github.com/gaozhihan/earth-forecasting-transformer/blob/5fb63264c5f241add4b75fd544e7dd78bcc5b401/scripts/cuboid_transformer/earthnet_w_meso/train_cuboid_earthnet.py#L33.
3. Modify the READMEs for the training scripts accordingly.
4. Add separate experiment dirs to .gitignore.