Remove partition strategy from tf.nn.safe_embedding_lookup_sparse and...

Remove partition strategy from tf.nn.safe_embedding_lookup_sparse and tf.nn.sampled_softmax_loss; use "div" partition strategy by default. We'll add support for different partition strategies later, probably in the PartitionedVariable class.

RELNOTES: We will use "div" as the default partition_strategy in tf.nn.safe_embedding_lookup_sparse, tf.nn.sampled_softmax and tf.nn.nce_loss.
PiperOrigin-RevId: 224257710