Implement log argument for suggest_int of ChainerMN integration.

[nzw0301]

Motivation

One of the follow-up PRs related to #1201

Description of the changes

• Introduce log argument for chainermn integration.
• Change low value to re-use test cases for suggest_int for simplicity.

[nzw0301]

@himkt Could you review this PR?

[codecov-commenter]

Codecov Report

Merging #1275 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1275      +/-   ##
==========================================
- Coverage   86.90%   86.73%   -0.18%     
==========================================
  Files          94       95       +1     
  Lines        7299     7438     +139     
==========================================
+ Hits         6343     6451     +108     
- Misses        956      987      +31     

Impacted Files	Coverage Δ
optuna/integration/chainermn.py	75.70% <ø> (+0.28%)	⬆️
optuna/cli.py	27.27% <0.00%> (-4.78%)	⬇️
optuna/storages/rdb/storage.py	95.16% <0.00%> (-0.67%)	⬇️
optuna/storages/base.py	67.92% <0.00%> (-0.30%)	⬇️
optuna/storages/cached_storage.py	95.63% <0.00%> (-0.20%)	⬇️
optuna/exceptions.py	100.00% <0.00%> (ø)
optuna/pruners/nop.py	75.00% <0.00%> (ø)
optuna/pruners/base.py	62.50% <0.00%> (ø)
optuna/samplers/base.py	47.05% <0.00%> (ø)
optuna/pruners/median.py	100.00% <0.00%> (ø)
... and 12 more

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[himkt]

Thank you for your constant contribution, it looks good.

[toshihikoyanase]

Thank you for your PR. It basically looks good to me.

It may be out of the scope of this PR, how about updating examples/chainermn_simple.py and examples/pruning/chainermn_integration.py?
I think we can simplify the suggestion of the number of units as follows:

- n_units = int(trial.suggest_loguniform("n_units_l{}".format(i), 4, 128))
+ n_units = trial.suggest_int("n_units_l{}".format(i), 4, 128, log=True)

[nzw0301]

@toshihikoyanase Thank you for your comments! It does not seem to be out of the scope for me. I have included updates of examples you suggested in this PR.

[toshihikoyanase]

Thank you for your update. LGTM!

[toshihikoyanase]

Let me update the title for the release note.