Fix/column annotation

[zhu0619]

Changelogs

• Add validator and serializer for content_type class
• Specified content_type to PDB in pdb_converter.

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Checklist:

• [ ] Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• [ ] Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
• [ ] Update the API documentation if a new function is added, or an existing one is deleted.
• [ ] Write concise and explanatory changelogs above.
• [ ] If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

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A minor PR to fix the dataset.table visualization and serialization for dataset content_type attribute.