Allows differentiating tfe.defun functions with loops in eager mode.

Adopts a minimal sensible policy for step containers: starting a graident tape
creates a step container; inner tapes do nothing; popping out of the outermost
tape will reset that step container. This should allow us to have reasonable
behavior in the presence of step-container-scoped things for a while. Ideally
we'll move away from them in favor of lists but the infrastructure isn't ready
yet.

PiperOrigin-RevId: 207911091