Improve annotations for _defaults_from_env_vars decorator

[rsokl]

What does this PR do?

This PR fixes the annotations for the the _defaults_from_env_vars decorator; this enables type-checkers to "see" the proper signature for Trainer.__init__.

mypy documents the proper annotation for a decorator, which I have incorporated here.

Before:
(The following screenshots are taken from VSCode with Pylance)

Signature of Trainer cannot be inspected by type-checkers:

IDEs cannot flag incorrect usage of Trainer:

After:

Signature is found!

Bad usages flagged!

Does your PR introduce any breaking changes? If yes, please list them.

None

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