Remove LearnerPipeline and LearnerPipelineConfig

[AdeelH]

Overview

This PR removes

• the legacy/unused LearnerPipeline and
• the LearnerPipelineConfig which was only used to wrap the LearnerConfig for saving it in the model bundle.

Other changes:

• The model bundles now store the LearnerConfig directly.
• The Config upgradation functionality which was previously limited to PipelineConfigs has been extended to all Configs.
• The ability to deserialize LearnerPipelineConfigs has been retained for backward compatibility.

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

Backward compatibility with older model bundles has been maintained.

Testing Instructions

See new/updated unit tests.

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Also related to #965

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9fd4dd5) 89.58% compared to head (e67e488) 89.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2056      +/-   ##
==========================================
+ Coverage   89.58%   89.68%   +0.09%     
==========================================
  Files         198      196       -2     
  Lines        9720     9723       +3     
==========================================
+ Hits         8708     8720      +12     
+ Misses       1012     1003       -9     

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