integrate memory saving gradients into dev
memory saving gradients for training networks (https://cmiclab.cs.ucl.ac.uk/CMIC/NiftyNet/tree/open_ai_checkpointing) should be made available at dev and in the pip release.
memory saving gradients for training networks (https://cmiclab.cs.ucl.ac.uk/CMIC/NiftyNet/tree/open_ai_checkpointing) should be made available at dev and in the pip release.