Remove Strategy.optimizer_zero_grad

[daniellepintz]

What does this PR do?

Remove Strategy.optimizer_zero_grad since no Strategy subclasses this hook, and we can just call the LM hook directly

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

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[awaelchli]

I disagree with the removal from Strategy.
The strategy can intercept the optimizer.step() so it should also be able to do that with the zero_grad() call imo.