Fix deprecated use of `sparse_to_dense`.

Calling `sparse_to_dense` gives a deprecation warning that asks users to use `sparse.to_dense`:
https://github.com/tensorflow/tensorflow/blob/71f40f044450736cd6acd29e92ffbfc0e571ee14/tensorflow/python/ops/sparse_ops.py#L952-L955

However, `sparse.to_dense` calls `sparse_to_dense`, which again produces the deprecation warning.