Find the frame with the caller on the stack.

def _find_frame(stack, start=0):
  """Find the frame with the caller on the stack."""
  # We want to find the first place where the layer was called
  # that is *not* an __init__ function of an inheriting layer.
  frame = inspect.getframeinfo(stack[start][0])
  # If we are in an init, move on.
  if frame.function == '__init__':
    return _find_frame(stack, start + 1)
  return frame