For compatibility purposes, when invoked in TF2 where the outermost context is eager mode, this API will check if there is a suitable TF2 summary writer context available, and if so will forward this call to that writer instead. A "suitable" writer context means that the writer is set as the default writer, and there is an associated non-empty value for step (seetf.summary.SummaryWriter.as_default, tf.summary.experimental.set_step or alternatively tf.compat.v1.train.create_global_step). For the forwarded call, the arguments here will be passed to the TF2 implementation oftf.summary.scalar, and the return value will be an empty bytestring tensor, to avoid duplicate summary writing. This forwarding is best-effort and not all arguments will be preserved.