Given a ndarray dict, find the optimal threshold for quantizing each value of the key.

def _get_optimal_thresholds(nd_dict, quantized_dtype, num_bins=8001, num_quantized_bins=255, logger=None):
    """Given a ndarray dict, find the optimal threshold for quantizing each value of the key."""
    if stats is None:
        raise ImportError('scipy.stats is required for running entropy mode of calculating'
                          ' the optimal thresholds for quantizing FP32 ndarrays into int8.'
                          ' Please check if the scipy python bindings are installed.')
    assert isinstance(nd_dict, dict)
    if logger is not None:
        logger.info('Calculating optimal thresholds for quantization using KL divergence'
                    ' with num_bins=%d and num_quantized_bins=%d' % (num_bins, num_quantized_bins))
    th_dict = {}
    # copy nd_dict keys since the keys() only returns a view in python3
    layer_names = list(nd_dict.keys())
    for name in layer_names:
        assert name in nd_dict
        min_val, max_val, min_divergence, opt_th = \
            _get_optimal_threshold(nd_dict[name], quantized_dtype, num_bins=num_bins,
                                   num_quantized_bins=num_quantized_bins)
        del nd_dict[name]  # release the memory of ndarray
        if min_val < 0:
            th_dict[name] = (-opt_th, opt_th)
        else:
            th_dict[name] = (0, opt_th)
        if logger is not None:
            logger.info('layer=%s, min_val=%f, max_val=%f, min_divergence=%f, optimal_threshold=%f'
                        % (name, min_val, max_val, min_divergence, opt_th))
    return th_dict