[WIP] Add commands for model training (#1203)

* Initial version of README for classification reference scripts

* More context

references/classification/README.md

+# Image classification reference training scripts
+
+This folder contains reference training scripts for image classification.
+They serve as a log of how to train specific models, as provide baseline
+training and evaluation scripts to quickly bootstrap research.
+
+Except otherwise noted, all models have been trained on 8x V100 GPUs.
+
+### ResNext-50 32x4d
+```
+python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
+    --model resnext50_32x4d --epochs 100
+```
+
+
+### ResNext-101 32x8d
+
+On 8 nodes, each with 8 GPUs (for a total of 64 GPUS)
+```
+python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
+    --model resnext101_32x8d --epochs 100
+```
+
+
+### MobileNetV2
+```
+python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
+     --model mobilenet_v2 --epochs 300 --lr 0.045 --wd 0.00004\
+     --lr-step-size 1 --lr-gamma 0.98
+```