[tune] Raise an error if Tuner.restore() is called on an instance

[krfricke]

Why are these changes needed?

Tuner.restore() is used to construct a Tuner object from an existing experiment. However, calling tuner = Tuner(...); tuner.restore() is a common misuse of the API and does not throw an error.

This PR updates the Tuner class to only allow restore() to be called on the class, not on an instance of the class.

Related issue number

Closes #39674

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[matthewdeng]

I wish there was a cleaner way to do this directly in the definition of restore, but I'm not sure if there is one...

[krfricke]

I don't think so, at least I can't think of one right now. The @classmethod decorator passes the class in both cases...

[matthewdeng]

Should this be TypeError instead?

[krfricke]

From https://docs.python.org/3/library/exceptions.html#AttributeError

exception AttributeError
Raised when an attribute reference (see Attribute references) or assignment fails. (When an object does not support attribute references or attribute assignments at all, TypeError is raised.)

And

https://docs.python.org/3/reference/expressions.html#attribute-references

If this attribute is not available, the exception AttributeError is raised.

I think AttributeError is correct. Tuner generally supports attribute references, but not this one. It's supposed to be like "this attribute does not exist for an instance (only for the class)"