Issue #590: rename log_dir to exp_name and log_root to log_dir

[mathpluscode]

Description

• Rename log_dir to exp_name
• Rename log_root to log_dir

Fixes #590

Type of change

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Code style update (formatting, renaming)
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• [x] Documentation Update (fix or improvement on the documentation)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Other (if none of the other choices apply)

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[codecov[bot]]

Codecov Report

Merging #698 (e660b3c) into main (f4422f9) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #698   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2424      2423    -1     
=========================================
- Hits          2424      2423    -1     

Impacted Files	Coverage Δ
deepreg/predict.py	100.00% <100.00%> (ø)
deepreg/train.py	100.00% <100.00%> (ø)
deepreg/util.py	100.00% <100.00%> (ø)

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[mathpluscode]

So I did

• Rename log_dir to exp_name
• Rename log_root to log_dir

It might be a bit confusing, but now

• log_dir tells you where all logs are saved
• exp_name let you identify each experiment/run

I've updated the docs correspondingly.

@YipengHu @NMontanaBrown @s-sd would you have time to review this?

[mathpluscode]

we have this docs currently

• Output directory:

  --log_dir, specifies the directory for saving all logs.

  By default, it is logs under the root of package.

  Example usage:

  • --log_dir logs for saving all logs under logs/.

• Experiment name:

  --exp_name or -n, specifies the name of the experiment, which will be used for specifying logs directory for the experiment.

  By default, it creates a timestamp-named directory under the log_dir, e.g. logs/20200810-194042/.

  Example usage:

  • --exp_name test for saving under logs/test/.

[YipengHu]

does log_dir has no shorthand '-l' now?