Create an interface for retrieving the current step number.

[copybara-service[bot]]

Create an interface for retrieving the current step number.

An implementation of this interface, when called for a training workload, will return the current step number, or 0 if training has not started.

This interface will be used for detecting workloads that hang, i.e., are not making progress. The hang detector will be implemented later.

Testing: test workloads.

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