A TensorRepresentationGroup is a collection of TensorRepresentations with
names. These names may serve as identifiers when converting the dataset
to a collection of Tensors or tf.CompositeTensors.
For example, given the following group:
{
  key: "dense_tensor"
  tensor_representation {
    dense_tensor {
      column_name: "univalent_feature"
      shape {
        dim {
          size: 1
        }
      }
      default_value {
        float_value: 0
      }
    }
  }
}
{
  key: "varlen_sparse_tensor"
  tensor_representation {
    varlen_sparse_tensor {
      column_name: "multivalent_feature"
    }
  }
}

Then the schema is expected to have feature "univalent_feature" and
"multivalent_feature", and when a batch of data is converted to Tensors using
this TensorRepresentationGroup, the result may be the following dict:
{
  "dense_tensor": tf.Tensor(...),
  "varlen_sparse_tensor": tf.SparseTensor(...),
}

Protobuf type tensorflow.metadata.v0.TensorRepresentationGroup