Fix `mypy` warnings about untyped functions

[Mr-Geekman]

🚀 Feature Request

Fix mypy warnings about untyped functions, because now it shows warnings like:

etna/transforms/decomposition/change_points_based/per_interval_models/constant.py:13: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]

Proposal

• Fix warnings
• Try to add option disallow-untyped-defs
  • It there aren't too many errors fix them and keep this option

Test cases

No response

Additional context

Developing in docs-prototype branch.

[Mr-Geekman]

Setting disallow_untyped_defs = true in pyproject.toml gives:

Found 358 errors in 99 files (checked 195 source files)

So, this issue requires a lot of changes. However, most of that I've seen is very simple.