Replace ForwardRef with TYPE_CHECKING

[RLKRo]

Description

When type import would cause cyclic import error, instead of defining types as ForwardRefs or pydantic's BaseModels, use TYPE_CHECKING.

This helps tools that work with typing such as mypy (this PR reduces the number of errors found with mypy from 208 to 145).

Checklist

• I have performed a self-review of the changes

To Consider

• Add tests (if functionality is changed)
• Update API reference / tutorials / guides
• Update CONTRIBUTING.md (if devel workflow is changed)
• Update .ignore files, scripts (such as lint), distribution manifest (if files are added/deleted)
• Search for references to changed entities in the codebase