Refactor `_tell.py`

[not522]

Motivation

The implementation of _tell.py is complicated, which causes bugs (#3808, #3814, #3819). This PR adds private methods and simplifies _tell_with_warning.

Description of the changes

• Add _get_frozen_trial and _check_state_and_values methods
• Change _check_and_convert_to_values to _check_values

[HideakiImamura]

@knshnb @contramundum53 Could you review this PR?

[contramundum53]

@HideakiImamura Should this be merged before v3?

[codecov-commenter]

Codecov Report

Merging #3841 (6f7dc7a) into master (ae7e309) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3841      +/-   ##
==========================================
+ Coverage   90.57%   90.60%   +0.02%     
==========================================
  Files         167      167              
  Lines       13111    13109       -2     
==========================================
+ Hits        11875    11877       +2     
+ Misses       1236     1232       -4     

Impacted Files	Coverage Δ
optuna/study/_optimize.py	98.44% <ø> (ø)
optuna/study/study.py	94.53% <ø> (ø)
optuna/study/_tell.py	100.00% <100.00%> (+2.00%)	:arrow_up:
optuna/samplers/_qmc.py	95.14% <0.00%> (+0.04%)	:arrow_up:
optuna/integration/botorch.py	98.25% <0.00%> (+0.87%)	:arrow_up:

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[not522]

Thank you for the detailed review. I addressed your comments. PTAL.

[knshnb]

Thanks for the update! Almost LGTM, but you dropped the cast to float of each element of values by this commit?

[not522]

Thank you. Fixed.