revise Pytorch_DDP README.md

Author: wkliao

examples/Pytorch_DDP/README.md

@@ -1,20 +1,27 @@
-# Example Python programs that use Pytorch Distributed Data Parallel module
+# Example Python programs that use Pytorch DDP module
 
 This directory contains example python programs that make use of Pytorch
-Distributed Data Parallel (DDP) module and MPI to run on multiple MPI processes
-in parallel. Detailed information describing the example programs is provided
-at the beginning of each file.
+Distributed Data Parallel
+([DDP](https://pytorch.org/tutorials/intermediate/ddp_tutorial.html)) module
+and [mpi4pi](https://mpi4py.readthedocs.io/en/stable/) to run on multiple MPI
+processes in parallel. Detailed information describing the example programs is
+provided at the beginning of each file.
 
-## [torch_ddp_skeleton.py](./torch_ddp_skeleton.py) shows how to set up the MPI
-and DDP environment to run a program in parallel.
+* [torch_ddp_skeleton.py](#torch_ddp_skeleton_py) -- a template for using
+  Pytorch DDP
 
-Command usage:
-```sh
-% mpiexec -n 4 python ./torch_ddp_skeleton.py 
-nprocs =  4  rank =  0  device =  cpu
-nprocs =  4  rank =  1  device =  cpu
-nprocs =  4  rank =  2  device =  cpu
-nprocs =  4  rank =  3  device =  cpu
-```
+---
 
+## torch_ddp_skeleton_py
+[torch_ddp_skeleton.py](./torch_ddp_skeleton.py) is a skeleton program showing
+how to set up the MPI and DDP environment to run a program in parallel.
+
+* Command usage and output on screen:
+  ```sh
+  % mpiexec -n 4 python ./torch_ddp_skeleton.py
+  nprocs =  4  rank =  0  device =  cpu
+  nprocs =  4  rank =  1  device =  cpu
+  nprocs =  4  rank =  2  device =  cpu
+  nprocs =  4  rank =  3  device =  cpu
+  ```