[Fix] Fix examples/distributed_training.py does not work in DDP (#700)

* Update distributed_training.py

Better example for DDP training

* Update distributed_training.py

* Update distributed_training.py

update according to reviwer's suggesstions.

* Update distributed_training.py

* Update distributed_training.py

The previous update copy data from main branch, its a mistake.
This update fix this mistake and the code is tested.