Make utils to handle the logic for threshold tuning objective and resplitting data

[tamargrey]

closes #3885
closes #3863

Allows us to determine for pipelines produced by automl search whether the full training data was used to train them.

[tamargrey]

I stuck these in automl utils instead of putting them in the engine_base file/making a new engine utils file, because part of their use is in determining after automl search whether pipelines were trained on the full data. So since they can be used in the wider context of automl, I thought it made sense to put them there. If anyone has any strong feelings to the contrary, definitely let me know!

[tamargrey]

The logic isn't changing, so I didn't add any tests that specifically test these utils on their own, but I'd be open to adding some if that's a necessary part of pulling this logic out into public utils

[codecov]

Codecov Report

Merging #3888 (9f7d393) into main (1ab688d) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3888     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        346     346             
  Lines      36352   36358      +6     
=======================================
+ Hits       36221   36227      +6     
  Misses       131     131             

Impacted Files	Coverage Δ
evalml/automl/__init__.py	100.0% <ø> (ø)
evalml/automl/engine/engine_base.py	100.0% <100.0%> (ø)
evalml/automl/utils.py	97.3% <100.0%> (+0.5%)	⬆️
evalml/tests/automl_tests/test_automl.py	99.5% <100.0%> (ø)
.../automl_tests/test_automl_search_classification.py	96.4% <100.0%> (ø)
evalml/tests/automl_tests/test_engine_base.py	100.0% <100.0%> (ø)

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[christopherbunn]

LGTM! RE: additional tests for the new functions, I also agree that it doesn't seem necessary given that the logic was covered before.

[eccabay]

Looks great, thanks for taking care of this!

[eccabay]

I have decided past tense release notes are the hill I'm going to die on. Can we do make --> made? Or even really "added" instead.

[tamargrey]

ah damn! I had noticed that and had been doing past tense to fit in. I'll make the change!

For what it's worth, my commit message (and release note since I mostly treat them the same) writing style comes from this article that I was forced asked to read at my first internship. The thing that made the biggest impression on me was this section

[tamargrey]

okay, I see the internet is kind of split between what tense release notes should be in, but more of them say past tense than present tense.

[eccabay]

Yeah, my personal ethos has always been past tense for release notes and present tense for commits, and that seems to be what we generally follow. Super interesting article though, lots to think about there!

[jeremyliweishih]

This LGTM as well - thanks for handling this!