fix: Fix HPO Grid Search comparison and name

[morelen17]

Issue #, if available:
-

Description of changes:

1. Replaced an identity-based strategy argument comparison with the GRID_SEARCH variable in the __init__ of the HyperparameterTuner class with the == comparison.
2. Renamed incorrect GridSearch hyperparameter tuning strategy to Grid (Amazon Sagemaker API Reference).

Testing done:
Existing unit tests already cover the issues.

Merge Checklist

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General

• I have read the CONTRIBUTING doc
• I certify that the changes I am introducing will be backward 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.

[knikure]

/bot run all

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-notebook-tests
• Commit ID: a7043e0
• Result: FAILED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: a7043e0
• Result: SUCCEEDED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-pr
• Commit ID: a7043e0
• Result: SUCCEEDED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: a7043e0
• Result: SUCCEEDED
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[codecov-commenter]

Codecov Report

Merging #3927 (a7043e0) into master (09aa2e1) will decrease coverage by 0.71%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3927      +/-   ##
==========================================
- Coverage   90.07%   89.37%   -0.71%     
==========================================
  Files        1156      271     -885     
  Lines      106848    26410   -80438     
==========================================
- Hits        96245    23603   -72642     
+ Misses      10603     2807    -7796     

Impacted Files	Coverage Δ
src/sagemaker/tuner.py	91.22% <100.00%> (ø)

... and 1426 files with indirect coverage changes

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: a7043e0
• Result: SUCCEEDED
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[knikure]

/bot run notebook-tests

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-notebook-tests
• Commit ID: a7043e0
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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