adding pytorch DDP script of detection task

references/detection/README.md

@@ -24,6 +24,27 @@ or PyTorch:
 ```shell
 python references/detection/train_pytorch.py db_resnet50 --train_path path/to/your/train_set --val_path path/to/your/val_set --epochs 5
 ```
+### Multi-GPU support (PyTorch only)
+
+Multi-GPU support on Detection task with PyTorch has been added. 
+Arguments are the same than the ones from single GPU, except:
+
+- `--devices`: **by default, if you do not pass `--devices`, it will use all GPUs on your computer**.
+You can use specific GPUs by passing a list of ids (ex: `0 1 2`). To find them, you can use the following snippet:
+
+```python
+import torch
+devices = [torch.cuda.device(i) for i in range(torch.cuda.device_count())]
+device_names = [torch.cuda.get_device_name(d) for d in devices]
+```
+
+- `--backend`: you can specify another `backend` for `DistribuedDataParallel` if the default one is not available on
+your operating system. Fastest one is `nccl` according to [PyTorch Documentation](https://pytorch.org/docs/stable/generated/torch.nn.parallel.DistributedDataParallel.html).
+
+
+```shell
+python references/detection/train_pytorch_ddp.py db_resnet50 --train_path path/to/your/train_set --val_path path/to/your/val_set --epochs 5 --devices 0 1 --backend nccl
+```
 
 ## Data format