code: def _make_histogram(values, bins):
      
      values = values.reshape(-1)
      counts, limits = np.histogram(values, bins=bins)
      limits = limits[1:]
  
      sum_sq = values.dot(values)
      return HistogramProto(min=values.min(),
                            max=values.max(),
                            num=len(values),
                            sum=values.sum(),
                            sum_squares=sum_sq,
                            bucket_limit=limits,
                            bucket=counts)
docs: Converts values into a histogram proto using logic from
      https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/lib/histogram/histogram.cc