Add Early Stopping to SageMaker Python SDK

[cassiesu]

Issue #, if available:

Description of changes:
Release SageMaker HPO EarlyStopping feature

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.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

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

[codecov-io]

Codecov Report

Merging #541 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
+ Coverage   92.77%   92.77%   +<.01%     
==========================================
  Files          71       71              
  Lines        5357     5359       +2     
==========================================
+ Hits         4970     4972       +2     
  Misses        387      387

Impacted Files	Coverage Δ
src/sagemaker/session.py	83.42% <ø> (ø)	⬆️
src/sagemaker/tuner.py	96.9% <100%> (+0.02%)	⬆️

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