!2551 dataset: repair bug in GetTensor that access NullPtr

Merge pull request !2551 from ms_yan/GetTensor_repair

mindspore/ccsrc/dataset/engine/datasetops/source/sampler/sampler.cc

@@ -91,11 +91,14 @@ void Sampler::Print(std::ostream &out, bool show_all) const {
 Status Sampler::GetAllIdsThenReset(py::array *data) {
   std::unique_ptr<DataBuffer> db;
   std::shared_ptr<Tensor> sample_ids;
+  TensorRow sample_row;
 
   // A call to derived class to get sample ids wrapped inside a buffer
   RETURN_IF_NOT_OK(GetNextSample(&db));
   // Get the only tensor inside the buffer that contains the actual SampleIds for the entire epoch
-  RETURN_IF_NOT_OK(db->GetTensor(&sample_ids, 0, 0));
+  RETURN_IF_NOT_OK(db->GetRow(0, &sample_row));
+  sample_ids = sample_row[0];
+
   // check this buffer is not a ctrl buffer
   CHECK_FAIL_RETURN_UNEXPECTED(db->buffer_flags() == DataBuffer::kDeBFlagNone, "ERROR ctrl buffer received");
   {

tests/ut/cpp/dataset/zip_op_test.cc

@@ -125,7 +125,6 @@ TEST_F(MindDataTestZipOp, MindDataTestZipOpDefault) {
     EXPECT_TRUE(rc.IsOk());
     row_count++;
   }
-  MS_LOG(WARNING) <<"row count is: " << row_count;
   ASSERT_EQ(row_count, 3); // Should be 3 rows fetched
 }