deepcam dummy wireup error

It's probably not a common use-case, but the "dummy" wireup method for deepcam doesn't seem to work.

Here's an example script at NERSC:

#!/bin/bash
#SBATCH -A nstaff_g
#SBATCH -q early_science
#SBATCH -C gpu
#SBATCH -J mlperf-deepcam
#SBATCH --nodes 1
#SBATCH --ntasks-per-node=1
#SBATCH --gpus-per-node 1
#SBATCH --cpus-per-task=32
#SBATCH --time 30
#SBATCH --image sfarrell/deepcam:ref-21.12

# Configuration
local_batch_size=2
batchnorm_group_size=1
data_dir="/global/cfs/cdirs/mpccc/gsharing/sfarrell/climate-data/All-Hist"
output_dir="$SCRATCH/deepcam/results"
run_tag="test_dummy_${SLURM_JOB_ID}"

srun --mpi=pmi2 shifter --module gpu \
       python ./train.py \
       --wireup_method "dummy" \
       --run_tag ${run_tag} \
       --data_dir_prefix ${data_dir} \
       --output_dir ${output_dir} \
       --model_prefix "segmentation" \
       --optimizer "LAMB" \
       --start_lr 0.0055 \
       --lr_schedule type="multistep",milestones="800",decay_rate="0.1" \
       --lr_warmup_steps 400 \
       --lr_warmup_factor 1. \
       --weight_decay 1e-2 \
       --logging_frequency 10 \
       --save_frequency 0 \
       --max_epochs 1 \
       --max_inter_threads 4 \
       --seed $(date +%s) \
       --batchnorm_group_size ${batchnorm_group_size} \
       --local_batch_size ${local_batch_size}

This gives a runtime error when constructing the DDP wrapper:

Traceback (most recent call last):
  File "./train.py", line 256, in <module>
    main(pargs)
  File "./train.py", line 167, in main
    ddp_net = DDP(net, device_ids=[device.index],
  File "/opt/conda/lib/python3.8/site-packages/torch/nn/parallel/distributed.py", line 551, in __init__
    self.process_group = _get_default_group()
  File "/opt/conda/lib/python3.8/site-packages/torch/distributed/distributed_c10d.py", line 412, in _get_default_group
    raise RuntimeError(
RuntimeError: Default process group has not been initialized, please make sure to call init_process_group.

Start with train.py main, especially the --wireup_method "dummy" path and the DDP construction at the reported line. Reproduce the NERSC command or its relevant arguments, then verify that the dummy run initializes the required process group and completes DDP construction without the reported runtime error.