add default config for RandomSubsetTrainingSampler in D2go

Summary:
Add default config for RandomSubsetTrainingSampler in D2 (https://github.com/facebookresearch/d2go/commit/3ee8885047e7ffb9eadcc6a1ecf8253c7ce9f79e)go.

User can use use the RandomSubsetTrainingSampler with the following yaml configs
```
DATALOADER:
  SAMPLER_TRAIN: RandomSubsetTrainingSampler
  RANDOM_SUBSET_RATIO: [Desired_ratio]  # for RandomSubsetTrainingSampler
```

Reviewed By: XiaoliangDai

Differential Revision: D29892366

fbshipit-source-id: cabb67fb46e51a93a8342a42f77a8a4d23a933e9

d2go/data/build.py

@@ -44,6 +44,14 @@ def add_weighted_training_sampler_default_configs(cfg: CfgNode):
     cfg.DATASETS.TRAIN_REPEAT_FACTOR = []
 
 
+def add_random_subset_training_sampler_default_configs(cfg: CfgNode):
+    """
+    Add default cfg.DATALOADER.RANDOM_SUBSET_RATIO for RandomSubsetTrainingSampler
+    The CfgNode under cfg.DATALOADER.RANDOM_SUBSET_RATIO should be a float > 0 and <= 1
+    """
+    cfg.DATALOADER.RANDOM_SUBSET_RATIO = 1.
+
+
 def get_train_datasets_repeat_factors(cfg: CfgNode) -> Dict[str, float]:
     repeat_factors = cfg.DATASETS.TRAIN_REPEAT_FACTOR
     assert all(len(tup) == 2 for tup in repeat_factors)

d2go/utils/get_default_cfg.py

@@ -3,6 +3,7 @@
 from d2go.config import CfgNode as CN
 from d2go.data.build import (
     add_weighted_training_sampler_default_configs,
+    add_random_subset_training_sampler_default_configs,
 )
 from d2go.data.config import add_d2go_data_default_configs
 from d2go.modeling.backbone.fbnet_cfg import (
@@ -52,6 +53,8 @@ def get_default_cfg(_C):
     add_quantization_default_configs(_C)
     # _C.DATASETS.TRAIN_REPEAT_FACTOR
     add_weighted_training_sampler_default_configs(_C)
+    # _C.DATALOADER.RANDOM_SUBSET_RATIO
+    add_random_subset_training_sampler_default_configs(_C)
     # _C.ABNORMAL_CHECKER
     add_abnormal_checker_configs(_C)
     # _C.MODEL.SUBCLASS