Fix assert wandb Run when mode="disabled"

[gautierdag]

What does this PR do?

This PR fixes #14021 which is caused by a bug introduced by the typing PR (#13483) which asserted that the wandb run object be a Run type and therefore breaks when mode="disabled" is used as wandb then uses the RunDisabled type. Unfortunately since our tests mock wandb, this behaviour was not tested against.

The fix is simply to allow for the RunDisabled object to be set under our logger _experiment attribute.

I've confirmed that the mock code in #14021 passes with this fix.

Fixes #14021

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

No

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