Programmatic way to get available training/inference containers.

**Describe the feature you'd like**
It would be very helpful to be able to programmatically get all available framerworks, py versions etc, prior to `sagemaker.image_uris.retrieve(...)`.
This would allow people to see what containers are available, without having to dig through repositories and try to find supported images or continuously hit errors until the right permutation of framework, framework version, and py version are achieved.

**How would this feature be used? Please describe.**
Something like `sagemaker.image_uris.show_images()` or something like that would be desired. This ideally would take no parameters and show maybe a dictionary of all the frameworks, framework versions, and respective py verisons.

**Describe alternatives you've considered**
Considering programmatically parsing: https://github.com/aws/deep-learning-containers/blob/master/available_images.md

But this does not feel like a good solution at all.

首先阅读 sagemaker.image_uris.retrieve(...) 入口点，并将其支持的 framework、framework-version 和 Python-version 组合与 deep-learning-containers/available_images.md 进行比较。确定无参数的 show_images() 风格 API 应如何表示所有可用组合，并考虑如何在不需要反复进行失败的 retrieval 的情况下，根据列出的容器验证完整性。

由索引模型根据 Issue 内容生成。