Fix warnings in `nn.sampled_softmax_loss` (#18494)

* Fix warnings in `nn.sampled_softmax_loss`

The softmax_cross_entropy_with_logits has been deprecated
and replaced with softmax_cross_entropy_with_logits_v2.
This causes nn.sampled_softmax_loss to always generate
a WANRING whenever called. This fix replaces
`softmax_cross_entropy_with_logits` with `softmax_cross_entropy_with_logits_v2`
and maintains the existing behavior to fix the warning.

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

* Pylint fix for line too long

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