Remove profile("training_step_and_backward")

[daniellepintz]

What does this PR do?

Remove profile("training_step_and_backward") in Closure class. It is not needed since we already profile the training_step and backward calls individually. Also remove _profiler reference from the class

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

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[awaelchli]

Remove TrainingTypePlugin.optimizer_zero_grad since no TTP subclasses this hook,

How do you know that? What would be the alternative? Wrap/patch the optimizer's zero_grad method? It's not clear to me why this removal is necessary

[daniellepintz]

How do you know that?

I grep'ed all the TTP classes for def optimizer_zero_grad and none of them override it.

What would be the alternative?

Users can override the LM's optimizer_zero_grad method

It's not clear to me why this removal is necessary

This removal isn't necessary, it is a simplification.

[daniellepintz]

Ready for merge!