Add to_png method and related test case

[tl3119]

Describe your changes

I have added the to_png method and the test case. Please check, and if there is any problem I need to fix, let me know!

Issue ticket number and link

Closes #x

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I have added thorough tests (when necessary).
• [ ] I have added the right documentation in the docstring and changelog (when needed)

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:books: Documentation preview :books:: https://sklearn-evaluation--303.org.readthedocs.build/en/303/

[coveralls]

Pull Request Test Coverage Report for Build 4279272723

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 94.485%

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Totals
Change from base Build 4236644410:	-0.6%
Covered Lines:	2741
Relevant Lines:	2901

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 4279272723

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.09%) to 94.02%

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Totals
Change from base Build 4236644410:	-1.09%
Covered Lines:	3050
Relevant Lines:	3244

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💛 - Coveralls