[BE] Spot leg animation

[jimmytyyang]

Motivation and Context

In the P0 demo and hab3 paper, we use leg animation to simulate Spot walking behavior. However, such a feature was never merged into the main. In addition, we receive a hab3 user request to add leg animation here: #1721

How Has This Been Tested

Run interactive play to visually verify the correctness

Types of changes

• [Development] A pull request that add new features to the habitat-lab task and environment codebase. Development Pull Requests must be small (less that 500 lines of code change), have unit testing, very extensive documentation and examples. These are typically new tasks, environments, sensors, etc... The review process for these Pull Request is longer because these changes will be maintained by our core team of developers, so make sure your changes are easy to understand!

Checklist

• [x] My code follows the code style of this project.
• [ ] I have updated the documentation if required.
• [x] I have read the CONTRIBUTING document.
• [x] I have completed my CLA (see CONTRIBUTING)
• [ ] I have added tests to cover my changes if required.

[jimmytyyang]

*the checkpoint of leg animation was uploaded when we did P0 demo

[ruddradev]

I found the working code present in the below merge request and was able to run the leg animation successfully -

https://github.com/facebookresearch/habitat-lab/pull/1554