Allow frozen dataclasses in apply_to_collection

[janEbert]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
  Docstring, yes.
• Did all existing and newly added tests pass locally?
  No, some tests were broken even without my changes. However, all tests in test_apply_func.py, including my new one, passed.

What does this PR do?

Allow frozen dataclasses to be ignored in apply_to_collection. This is useful to allow ignoring values which we never cared about anyway, for example when using apply_to_collection to move data to a device.
This (combined with a change in the previously linked line, see main repo PR) allows saving optimizers for which the state includes frozen dataclasses that we just want to pass through.

Should be completely backward-compatible since I introduce a new keyword argument for this.