[FEATURE] Add multi gpu example using pytorch ddp

[YilingQiao]

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Description

Add an example for running genesis+torch on multi-gpu. Try

torchrun --standalone --nnodes=1 --nproc_per_node=2 examples/ddp_multi_gpu.py

Related Issue

Resolves Genesis-Embodied-AI/Genesis#

Motivation and Context

How Has This Been / Can This Be Tested?

Screenshots (if appropriate):

Checklist:

• [x] I read the CONTRIBUTING document.
• [x] I followed the Submitting Code Changes section of CONTRIBUTING document.
• [x] I tagged the title correctly (including BUG FIX/FEATURE/MISC/BREAKING)
• [x] I updated the documentation accordingly or no change is needed.
• [x] I tested my changes and added instructions on how to test it for reviewers.

• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.