Issue #585: add example for custom loss

[mathpluscode]

Description

Fixes #585

Type of change

What types of changes does your code introduce to DeepReg?

Please check the boxes that apply after submitting the pull request.

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [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)

Checklist

Please check the boxes that apply after submitting the pull request.

If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I have installed pre-commit using pre-commit install and formatted all changed files. If you are not certain, run pre-commit run --all-files.
• [x] My commits' message styles matches our requested structure, e.g. Issue #<issue number>: detailed message.
• [x] I have updated the change log file regarding my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

Merging #687 (af993fb) into main (21e2a1c) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #687   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2432      2432           
=========================================
  Hits          2432      2432           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 21e2a1c...af993fb. Read the comment docs.

[mathpluscode]

@YipengHu I've updated the docs, please have a second look ^^