Changing KeyValueTensorInitializer to be more general.

This is similar to the previous version but replaces the initialize_table_v2 call with gen_lookup_ops.lookup_table_import_v2. They are conceptually doing the same thing but initialize_table_v2 requires table to be a child of InitializableLookupTable whereas lookup_table_import_v2 does not. So the new intializer is more general than previous one.

PiperOrigin-RevId: 210182320