* adding create checkpoint feature for train function in farm reader
* added arguments for create_or_load_checkpoint function
* accessing class method inside Trainer class
* added default value for checkpoint_root_dir and checkpoint_every, checkpoints_to_keep as arguments for reader.train()
* change in default value for checkpoint_root_dir and checkpoint_every
* update docstring and add Path conversion
Co-authored-by: girish.koushik <girish.koushik@diatoz.com>
Co-authored-by: Malte Pietsch <malte.pietsch@deepset.ai>