The sagemaker model object URI is over-written when calling the compile API

[kkoppolu1]

Describe the bug
The sagemaker Model object can be created by supplying the image_uri attribute. However, when the compile API is invoked on the object, the image URI gets over-written. The overwrite is happening at https://github.com/aws/sagemaker-python-sdk/blob/master/src/sagemaker/model.py#L403.

To reproduce

1. Create a sagemaker Model object like MXNetModel or PyTorchModel with an image_uri set.
2. Invoke compile with target instance from the "ml" family (eg ml_c5).
3. Inspect the image URI of the object.
   NOTE: If it is the same, it maybe because the value you chose is the same as the value being set. Try another value for the image_uri.

Expected behavior
When the model is created with a specific image_uri, it is expected that the model adheres to it and does not overwrite it.

Screenshots or logs
If applicable, add screenshots or logs to help explain your problem.

System information
A description of your system. Please provide:

• SageMaker Python SDK version: 2.0
• Framework name (eg. PyTorch) or algorithm (eg. KMeans): MXNet and PyTorch. Could impact other frameworks.
• Framework version: MXNet: 1.7. PyTorch: 1.4
• Python version: py3
• CPU or GPU: CPU. Impacts GPU as well.
• Custom Docker image (Y/N): N

Additional context
Add any other context about the problem here.

[ajaykarpur]

Thanks @kkoppolu1, this does indeed look like a bug. We'll have a fix out for this shortly.

It actually appears that this is the intended behavior:

sagemaker-python-sdk/tests/unit/sagemaker/model/test_neo.py

Lines 55 to 59 in be554f4

	assert (
	"{}.dkr.ecr.{}.amazonaws.com/sagemaker-neo-tensorflow:1.15.0-inf-py3".format(
	NEO_REGION_ACCOUNT, REGION
	)
	== model.image_uri

[ajaykarpur]

This was introduced in #1373: https://github.com/aws/sagemaker-python-sdk/pull/1373/files#diff-5d463823465839ff530ad10c7dfc0593R386-R393

[ajaykarpur]

Also related: #1938