azureml-defaults not described

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We need details description of `azureml-defaults`.

We need this when deployment. In training, we usually use `azureml-core`. In deployment, `azureml-defaults` is necessary (only `azureml-core` is not enough to deploy). I heard `azureml-defaults` includes `azureml-core`. But it is not documented.

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#### Document Details

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* ID: 8e0e12a4-b363-2726-06b4-9db2015efb32
* Version Independent ID: e39a91ac-375b-a2cc-350d-a82cb7b0b035
* Content: [Install the Azure Machine Learning SDK for Python - Azure Machine Learning Python](https://docs.microsoft.com/en-us/python/api/overview/azure/ml/install?view=azure-ml-py)
* Content Source: [AzureML-Docset/docs-ref-conceptual/install.md](https://github.com/MicrosoftDocs/MachineLearning-Python-pr/blob/live/AzureML-Docset/docs-ref-conceptual/install.md)
* Service: **machine-learning**
* Sub-service: **core**
* GitHub Login: @harneetvirk
* Microsoft Alias: **harnvir**