Add YOLOX support

[FANGAreNotGnu]

• Add support for YOLOX (yolox-s and yolox-l)
• Update finetuning example to include yolox
• Update code loading customized detection configs: store locally in multimodal/configs/pretrain/detection instead of download from s3.
• Add max_tuning_num in fit() to control the max number of data to use for validation.
• Add clean_old_ckpts flag to control if we shall clean the old ckpts.

YoloX-L performance on pothole dataset:
training time: 1337 seconds
mAP: 0.449
mAP50: 0.736

Previous best (VFNet-R50):
training time: ~18000 seconds
mAP: 0.450
mAP50: 0.738

TODO:

• Test the YOLOX pretrained on objects365
• More pretrain tunings.
• Test YOLOX-tiny/s/m/x
• update tutorial to use YOLOX

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[github-actions]

Job PR-2644-08a8c29 is done.
Docs are uploaded to http://autogluon-staging.s3-website-us-west-2.amazonaws.com/PR-2644/08a8c29/index.html

[github-actions]

Job PR-2644-6ec55ff is done.
Docs are uploaded to http://autogluon-staging.s3-website-us-west-2.amazonaws.com/PR-2644/6ec55ff/index.html

[github-actions]

Job PR-2644-5079fd5 is done.
Docs are uploaded to http://autogluon-staging.s3-website-us-west-2.amazonaws.com/PR-2644/5079fd5/index.html

[bryanyzhu]

LGTM, thanks for bringing yolox in.

[zhiqiangdon]

LGTM!