def tensor_vstack(tensor_list, pad=0):
    """
    vertically stack tensors by adding a new axis
    expand dims if only 1 tensor
    :param tensor_list: list of tensor to be stacked vertically
    :param pad: label to pad with
    :return: tensor with max shape
    """
    if len(tensor_list) == 1:
        return tensor_list[0][np.newaxis, :]

    ndim = len(tensor_list[0].shape)
    dimensions = [len(tensor_list)]  # first dim is batch size
    for dim in range(ndim):
        dimensions.append(max([tensor.shape[dim] for tensor in tensor_list]))

    dtype = tensor_list[0].dtype
    if pad == 0:
        all_tensor = np.zeros(tuple(dimensions), dtype=dtype)
    elif pad == 1:
        all_tensor = np.ones(tuple(dimensions), dtype=dtype)
    else:
        all_tensor = np.full(tuple(dimensions), pad, dtype=dtype)
    if ndim == 1:
        for ind, tensor in enumerate(tensor_list):
            all_tensor[ind, :tensor.shape[0]] = tensor
    elif ndim == 2:
        for ind, tensor in enumerate(tensor_list):
            all_tensor[ind, :tensor.shape[0], :tensor.shape[1]] = tensor
    elif ndim == 3:
        for ind, tensor in enumerate(tensor_list):
            all_tensor[ind, :tensor.shape[0], :tensor.shape[1], :tensor.shape[2]] = tensor
    else:
        raise Exception('Sorry, unimplemented.')
    return all_tensor
vertically stack tensors by adding a new axis
    expand dims if only 1 tensor
    :param tensor_list: list of tensor to be stacked vertically
    :param pad: label to pad with
    :return: tensor with max shape