Verifies that a global step tensor is valid or gets one if None is given.
tf.contrib.framework.assert_or_get_global_step( graph=None, global_step_tensor=None )
global_step_tensor is not None, check that it is a valid global step
assert_global_step). Otherwise find a global step tensor using
get_global_step and return it.
graph: The graph to find the global step tensor for.
global_step_tensor: The tensor to check for suitability as a global step. If None is given (the default), find a global step tensor.
A tensor suitable as a global step, or
None if none was provided and none