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

self.self_attention = attention_spec.MultiHeadAttentionSpec(self_attention=True)
        self.ffn = FeedForwardSpec()

class TransformerDecoderLayerSpec(model_spec.LayerSpec):
    def __init__(self):
        self.self_attention = attention_spec.MultiHeadAttentionSpec(self_attention=True)
        self.attention = attention_spec.MultiHeadAttentionSpec()
        self.ffn = FeedForwardSpec()

class FeedForwardSpec(model_spec.LayerSpec):
    def __init__(self):
        self.layer_norm = common_spec.LayerNormSpec()
        self.linear_0 = common_spec.LinearSpec()
        self.linear_1 = common_spec.LinearSpec()

class PositionEncoderSpec(model_spec.LayerSpec):
    def __init__(self):
        self.encodings = model_spec.OPTIONAL