Add broadcast support for softmax_cross_entropy_with_logits (#16784)

* Add broadcast support for softmax_cross_entropy_with_logits

This fix tries to address the issue raised in 11534 where
there was no broadcast support for SoftmaxCrossEntropyWithLogits.
This fix adds the broadcast support for SoftmaxCrossEntropyWithLogits,
and adds test cases for it.

This fix fixes 11534.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add BroadcastBinaryOpOutputShapeFn for shape function

This commit adds BroadcastBinaryOpOutputShapeFn,
so that the implementation of BroadcastBinaryOpShapeFn
coule be reused in SoftmaxCrossEntropyWithLogits.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update the shape function of SoftmaxCrossEntropyWithLogits

so that broadcast could be supported.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add broadcast support for SoftmaxCrossEntropyWithLogits

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add broadcast support for SoftmaxCrossEntropyWithLogits with GPU

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Reformat with clang-format

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Fix shape test issues

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Remove `_` for gen_nn_ops._softmax_cross_entropy_with_logits

as `_` is not needed anymore with the recent changes

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Sanitize nn_ops.cc with clang-format

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add broadcast examples for SoftmaxCrossEntropyWithLogits shape function

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add benchmark tests for trival cases

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Fix pylint issue

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>