Fix type hint for train_dataset param of Trainer.__init__() to allow IterableDataset. Issue 29678

[stevemadere]

Prevents introducing confusion for developers using PyRight and other type checking
systems when they try to use Trainer.train() as documented with an IterableDataset
for training.

No new dependencies.

Fixes #29678

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@muellerzr , @pacman100

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[amyeroberts]

Thanks for adding!

The failing test is unrelated to this PR - there was a fix pushed to main, could you try rebasing to include this update?

[muellerzr]

Thanks! Agreed, we can merge once the ci is fully green (unrelated to you though)