Migrate from std::string to tensorflow::tstring. (#705)

Note that during the transition period tstring is typedef'ed to
std::string.

See: tensorflow/community#91

Author: gharibian

tensorflow_addons/custom_ops/text/cc/kernels/skip_gram_kernels.cc

@@ -133,12 +133,12 @@ class SkipGramGenerateCandidatesOp : public OpKernel {
                               .TypeConstraint<type>("T"),           \
                           SkipGramGenerateCandidatesOp<type>)
 
-REGISTER_KERNEL(string);
+REGISTER_KERNEL(tstring);
 REGISTER_KERNEL(int64);
 REGISTER_KERNEL(int32);
 REGISTER_KERNEL(int16);
 
 #undef REGISTER_KERNEL
 
 }  // end namespace addons
-}  // namespace tensorflow
+}  // namespace tensorflow