Add a check for Key being scalar tensor for MapStage and OrderedMapStage ops.

According to documentation[1][2], key must be int64 value, but this wasn't enforced and the ops would fail with check failure for non-scalar key value.

[1]https://www.tensorflow.org/api_docs/cc/class/tensorflow/ops/ordered-map-stage
[2]https://www.tensorflow.org/api_docs/cc/class/tensorflow/ops/map-stage

PiperOrigin-RevId: 413822112
Change-Id: I9d118faf990e6361900aa32272eff486ad9f0e2e