message: Restore 0-arg version of Tensor::DebugString.
diff: @@ -529,7 +529,8 @@ class Tensor {
  // `num_values` is the number of actual data values in the tensor
  // included in the message. If the tensor might be resident in
  // GPU/TPU memory use DeviceSafeDebugString instead.
  - string DebugString(int num_values = 3) const;
  + string DebugString(int num_values) const;
  + string DebugString() const { return DebugString(3); }
  // Variant of DebugString() that should be used for possibly non-CPU tensors.
  // If the tensor is not resident on CPU, we can't read its values as
  