Multigrid Charades with only 1 GPU

Hi, I downloaded the Charades dataset and tried to train the dataset with the command:
`python tools/run_net.py --cfg configs/Charades/SLOWFAST_16x8_R50_multigrid.yaml DATA.PATH_TO_DATA_DIR ../Charades_v1_rgb/
`

Considering that I've only 1 GPU, I edited the .yaml NUM_GPUS parameter to 1.
So the .yaml file configuration is:
```yaml
MULTIGRID:
SHORT_CYCLE: True
LONG_CYCLE: True
TRAIN:
ENABLE: True
DATASET: charades
BATCH_SIZE: 16
EVAL_PERIOD: 6
CHECKPOINT_PERIOD: 6
AUTO_RESUME: True
CHECKPOINT_FILE_PATH: SLOWFAST_16x8_R50_multigrid.pkl
CHECKPOINT_TYPE: pytorch
DATA:
NUM_FRAMES: 64
SAMPLING_RATE: 2
TRAIN_JITTER_SCALES: [256, 340]
TRAIN_CROP_SIZE: 224
TEST_CROP_SIZE: 256
INPUT_CHANNEL_NUM: [3, 3]
MULTI_LABEL: True
INV_UNIFORM_SAMPLE: True
ENSEMBLE_METHOD: max
REVERSE_INPUT_CHANNEL: True
SLOWFAST:
ALPHA: 4
BETA_INV: 8
FUSION_CONV_CHANNEL_RATIO: 2
FUSION_KERNEL_SZ: 7
RESNET:
SPATIAL_STRIDES: [[1, 1], [2, 2], [2, 2], [2, 2]]
SPATIAL_DILATIONS: [[1, 1], [1, 1], [1, 1], [1, 1]]
ZERO_INIT_FINAL_BN: True
WIDTH_PER_GROUP: 64
NUM_GROUPS: 1
DEPTH: 50
TRANS_FUNC: bottleneck_transform
STRIDE_1X1: False
NUM_BLOCK_TEMP_KERNEL: [[3, 3], [4, 4], [6, 6], [3, 3]]
NONLOCAL:
LOCATION: [[[], []], [[], []], [[], []], [[], []]]
GROUP: [[1, 1], [1, 1], [1, 1], [1, 1]]
INSTANTIATION: dot_product
BN:
USE_PRECISE_STATS: True
NUM_BATCHES_PRECISE: 200
NORM_TYPE: sync_batchnorm
NUM_SYNC_DEVICES: 4
SOLVER:
BASE_LR: 0.0375
LR_POLICY: steps_with_relative_lrs
LRS: [1, 0.1, 0.01, 0.001, 0.0001, 0.00001]
STEPS: [0, 41, 49]
MAX_EPOCH: 57
MOMENTUM: 0.9
WEIGHT_DECAY: 1e-4
WARMUP_EPOCHS: 4.0
WARMUP_START_LR: 0.0001
OPTIMIZING_METHOD: sgd
MODEL:
NUM_CLASSES: 157
ARCH: slowfast
LOSS_FUNC: bce_logit
HEAD_ACT: sigmoid
DROPOUT_RATE: 0.5
TEST:
ENABLE: True
DATASET: charades
BATCH_SIZE: 16
NUM_ENSEMBLE_VIEWS: 10
NUM_SPATIAL_CROPS: 3
DATA_LOADER:
NUM_WORKERS: 8
PIN_MEMORY: True
NUM_GPUS: 1
NUM_SHARDS: 1
RNG_SEED: 0
OUTPUT_DIR: .
LOG_MODEL_INFO: False
```
Running the command, I get this error:
```
File "tools/run_net.py", line 42, in
main()
File "tools/run_net.py", line 23, in main
launch_job(cfg=cfg, init_method=args.init_method, func=train)
File "/home/rscavo/SlowFast/slowfast/utils/misc.py", line 296, in launch_job
func(cfg=cfg)
File "/home/rscavo/SlowFast/tools/train_net.py", line 393, in train
train_loader = loader.construct_loader(cfg, "train")
File "/home/rscavo/SlowFast/slowfast/datasets/loader.py", line 88, in construct_loader
batch_sampler = ShortCycleBatchSampler(
File "/home/rscavo/SlowFast/slowfast/datasets/multigrid_helper.py", line 20, in __init__
raise ValueError(
ValueError: sampler should be an instance of torch.utils.data.Sampler, but got sampler=None
```

The sampler is created using the create_sampler function:
https://github.com/facebookresearch/SlowFast/blob/fd41618191d3c21c1ca21a61369ce9917646cf9c/slowfast/datasets/loader.py#L87

the create_sampler is defined as follows:
https://github.com/facebookresearch/SlowFast/blob/fd41618191d3c21c1ca21a61369ce9917646cf9c/slowfast/datasets/utils.py#L304-L318

I tried to change
``` python
cfg.NUM_GPUS > 1
```
to
``` python
cfg.NUM_GPUS > 0
```
Doing that the error changed in:
```
File "tools/run_net.py", line 42, in
main()
File "tools/run_net.py", line 23, in main
launch_job(cfg=cfg, init_method=args.init_method, func=train)
File "/home/rscavo/SlowFast/slowfast/utils/misc.py", line 296, in launch_job
func(cfg=cfg)
File "/home/rscavo/SlowFast/tools/train_net.py", line 393, in train
train_loader = loader.construct_loader(cfg, "train")
File "/home/rscavo/SlowFast/slowfast/datasets/loader.py", line 87, in construct_loader
sampler = utils.create_sampler(dataset, shuffle, cfg)
File "/home/rscavo/SlowFast/slowfast/datasets/utils.py", line 316, in create_sampler
sampler = DistributedSampler(dataset) if cfg.NUM_GPUS > 0 else None
File "/home/rscavo/anaconda3/lib/python3.8/site-packages/torch/utils/data/distributed.py", line 54, in __init__
num_replicas = dist.get_world_size()
File "/home/rscavo/anaconda3/lib/python3.8/site-packages/torch/distributed/distributed_c10d.py", line 620, in get_world_size
return _get_group_size(group)
File "/home/rscavo/anaconda3/lib/python3.8/site-packages/torch/distributed/distributed_c10d.py", line 219, in _get_group_size
_check_default_pg()
File "/home/rscavo/anaconda3/lib/python3.8/site-packages/torch/distributed/distributed_c10d.py", line 209, in _check_default_pg
assert _default_pg is not None, \
AssertionError: Default process group is not initialized
```
Do you know a way to be able to use SlowFast with only one GPU or am I doing something incorrectly? Thank you!