Add missing type hint to EncodedVideo.from_path()'s return value

[abstractionmage]

Motivation and Context

If a user loads a video through the EncodedVideo.from_path method, they will not get any type hints for that video in their IDE because that method does not specify its return type. This PR fixes that.

How Has This Been Tested

The method was called from another Python script. Since I just added a type hint and no syntax errors were raised, there's no possibility for anything to have broken.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• [not necessary] I have added tests to cover my changes.
• [not necessary] All new and existing tests passed.