Sort params on fetch

[contramundum53]

Motivation

On RDBStorage, param order is not conserved in user API, although the database stores the order information in the form of param_id. This creates issues (e.g. #4771) for certain samplers that use the order of parameters to analyze the parameter dependency relation in dynamic search space. We ensure that RDBStorage returns the parameters in the order they are suggested.

Description of the changes

• Ensure that trial.params is in the order they are suggested, in RDBStorage.
• Test that for all storages, trial.params are in the right order.

[not522]

@gen740 Could you review this PR?

[contramundum53]

After a discussion, we decided not to require all storage classes to preserve the order of parameters and now #4771 can be solved independently by #4782. However, preserving the order of parameters is still helpful when it's possible, so I'll leave this PR open.

[codecov-commenter]

Codecov Report

Merging #4775 (22d5f92) into master (1b0c6a8) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #4775      +/-   ##
==========================================
- Coverage   90.75%   90.73%   -0.02%     
==========================================
  Files         190      190              
  Lines       14468    14470       +2     
==========================================
  Hits        13130    13130              
- Misses       1338     1340       +2     

Impacted Files	Coverage Δ
optuna/storages/_rdb/storage.py	94.33% <100.00%> (+0.02%)	⬆️

... and 2 files with indirect coverage changes

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[not522]

LGTM

[gen740]

LGTM!