Cleanup diskspace for docker image build

[yinweisu]

Issue #, if available:

Description of changes:

• Nightly image building is failing recently because of the increase of the image size leading to out of disk space for github runners. Adding a step to cleanup the machine to squeeze more space.
• Has been tested here
• To be noticed, if our image size continue to grow, it's likely that we'll need to move away from github hosted runners...

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[tonyhoo]

Looks good to me, a more concerning issue is the root cause of ever growing image size

[yinweisu]

Looks good to me, a more concerning issue is the root cause of ever growing image size

I think we are just keep adding more dependencies, mostly in multimodal

[github-actions]

Job PR-3283-c374418 is done.
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[github-actions]

Job PR-3283-5e60397 is done.
Docs are uploaded to http://autogluon-staging.s3-website-us-west-2.amazonaws.com/PR-3283/5e60397/index.html