[Core] Create global-step increment op

[michaelschaarschmidt]

Global step is created in the executor and used for checkpointing. An increment op would need to be created e.g. in the generic agent and called together with act.

[sven1977]

This has been done for training updates, but not for act updates (which would need a separate counter).
The training counter variable is held by the graph_executor under self.global_training_timestep and must be increased by Agents by calling self._graph_fn_training_step (which returns the increment up and may take another op for control dependencies). All current Agents execute this call.