change: make image_scope optional for some images in image_uris.retrieve()

[laurenyu]

Issue #, if available:
#1464

Description of changes:
This was common enough that I decided to go ahead and add logic for the case where a single image is used for both training and inference. This should also hopefully help with making it easier to migrate from sagemaker.amazon.amazon_estimator.get_image_uri to sagemaker.image_uris.retrieve

Testing done:
unit tests

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• I have read the CONTRIBUTING doc
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Tests

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[sagemaker-bot]

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