How to use the ctranslate2.specs.model_spec.ModelSpec function in ctranslate2

"""Declares specification of the Transformer model."""

import numpy as np

from ctranslate2.specs import attention_spec
from ctranslate2.specs import common_spec
from ctranslate2.specs import model_spec


class TransformerSpec(model_spec.ModelSpec):
    """Describes a Transformer model.

    The specification is invariant to hidden dimensions but requires to
    explicitly set the number of layers and attention heads.
    """
    def __init__(self, num_layers, num_heads):
        self.num_heads = np.dtype("int8").type(num_heads)
        self.encoder = TransformerEncoderSpec(num_layers)
        self.decoder = TransformerDecoderSpec(num_layers)

    @property
    def revision(self):
        return 3

    @property
    def source_vocabulary_size(self):