Adding S3 capabilities
This MR adds several capabilities for training with S3 data storage support:
- download files from S3 (parallel processing),
- checkpoint the model and performance on s3 (need to use lightning.pytorch.loggers.TensorBoardLogger instead of Comet).
This is required for long-term training on ml.cern.ch (due to 24h limit of Kerberos token for Eos access).