Migrate from std::string to tensorflow::tstring.

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

See: tensorflow/community#91
PiperOrigin-RevId: 282043204

tensor2tensor/data_generators/ops/subword_text_encoder_ops.cc

@@ -14,6 +14,7 @@ using ::tensorflow::OpKernelContext;
 using ::tensorflow::Status;
 using ::tensorflow::Tensor;
 using ::tensorflow::TensorShape;
+using ::tensorflow::tstring;
 using ::tensorflow::shape_inference::InferenceContext;
 
 REGISTER_OP("SubwordTextEncoderEncode")
@@ -36,7 +37,7 @@ class SubwordTextEncoderEncodeOp : public OpKernel {
 
   void Compute(OpKernelContext* ctx) override {
     // Get input string and deserialize into ArticleExample proto.
-    const string& s = ctx->input(0).scalar<string>()();
+    absl::string_view s = ctx->input(0).scalar<tstring>()();
 
     // Construct encoded output tensors.
     std::vector<int> encoded_ids;