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 (see
tf.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 of
tf.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.