doc: add documentation for image_uri serverless use case

[bhaoz]

Issue #, if available:

Description of changes:
Add more doc to explain using image_uri in Serverless inference

Testing done:
Unit tests and docstyle tests

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• I have read the CONTRIBUTING doc
• I certify that the changes I am introducing will be backword compatible, and I have discussed concerns about this, if any, with the Python SDK team
• I used the commit message format described in CONTRIBUTING
• I have passed the region in to all S3 and STS clients that I've initialized as part of this change.
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added unit and/or integration tests as appropriate to ensure backward compatibility of the changes
• I have checked that my tests are not configured for a specific region or account (if appropriate)
• I have used unique_name_from_base to create resource names in integ tests (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #3022 (5f6282e) into master (b0f2b4a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3022   +/-   ##
=======================================
  Coverage   89.32%   89.33%           
=======================================
  Files         196      196           
  Lines       16622    16622           
=======================================
+ Hits        14848    14849    +1     
+ Misses       1774     1773    -1     

Impacted Files	Coverage Δ
src/sagemaker/chainer/model.py	97.56% <ø> (ø)
src/sagemaker/huggingface/model.py	79.10% <ø> (ø)
src/sagemaker/mxnet/model.py	96.36% <ø> (ø)
src/sagemaker/pytorch/model.py	85.45% <ø> (ø)
src/sagemaker/sklearn/model.py	81.63% <ø> (ø)
src/sagemaker/tensorflow/model.py	90.21% <ø> (ø)
src/sagemaker/xgboost/model.py	97.43% <ø> (ø)
src/sagemaker/session.py	70.47% <0.00%> (+0.07%)	⬆️

---

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: 2a91b30
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: 2a91b30
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: 2a91b30
• Result: FAILED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: 965bbad
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: 965bbad
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: 965bbad
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: 5f6282e
• Result: FAILED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: 5f6282e
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: 5f6282e
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: 5f6282e
• Result: FAILED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: 5f6282e
• Result: FAILED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: 5f6282e
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository