Improve typehints and docs in diffusers/models

[a-r-r-o-w]

What does this PR do?

Fixes #5249.

Improves type hints and docs in the following files:

• diffusers/models/activations.py
• diffusers/models/resnet.py

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

@patrickvonplaten @yiyixuxu

[yiyixuxu]

thanks!