message: Make sure the `NullPrefetchCallback` always does some work, even when it is not prefetched.
  Summary: This is useful for producing accurate comparisons of the input data feeds.
  Test Plan: Existing CI tests
  Reviewers: #tensorflow, davidn
diff: @@ -362,7 +362,11 @@ class NullPrefetchCallback : public poplar::StreamCallback {
  return poplar::StreamCallback::Result::Success;
  }
  - void fetch(void* dest) noexcept override {}
  + void fetch(void* dest) noexcept override {
  + // This case shouldn't be hit, if poplar prefetches the data.
  + std::memcpy(dest, buffer.data(), num_bytes_);
  + }
  +
  void complete() noexcept override {}
  private:
  