dyad: update multiprocessing approach for DLIO

This commit tweaks the DLIO config file to use forking for
multiprocessing instead of spawning

Author: ilumsden

2024-HPCIC-AWS/JupyterNotebook/tutorial/supplementary/dyad/dlio_extensions/configs/workload/dyad_unet3d_demo.yaml

@@ -20,7 +20,7 @@ reader:
   read_threads: 1
   file_shuffle: seed
   sample_shuffle: seed
-  multiprocessing_context: spawn
+  multiprocessing_context: fork
   data_loader_classname: dyad_torch_data_loader.DyadTorchDataLoader
   data_loader_sampler: index
 
@@ -32,4 +32,4 @@ checkpoint:
   checkpoint_folder: checkpoints/unet3d
   checkpoint_after_epoch: 5
   epochs_between_checkpoints: 2
-  model_size: 499153191
+  model_size: 499153191