[Misc] Infer model name from model_uri and check AWS credential

[brosoul]

Pull Request Description

1. Infer model name from model_uri and update related docs, address DOWNLOADER_MODEL_NAME blocks the runtime to download other models #212
2. Check the existence of AWS credential, address aibrix throw AttributeError if the credentials is missed #208

Related Issues

Resolves: #212, #208

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[Jeffwan]

nit, --model-name is still a little bit confused, --model-folder-name makes more sense for a downloader library. this is not feature/bug related and we can improve it later