src/transformers/models/wav2vec2/modeling_wav2vec2.py/Wav2Vec2Adapter/__init__
class Wav2Vec2Adapter:    def __init__(self, config):
        super().__init__()

        # feature dim might need to be down-projected
        if config.output_hidden_size != config.hidden_size:
            self.proj = nn.Linear(config.hidden_size, config.output_hidden_size)
            self.proj_layer_norm = nn.LayerNorm(config.output_hidden_size)
        else:
            self.proj = self.proj_layer_norm = None

        self.layers = nn.ModuleList(Wav2Vec2AdapterLayer(config) for _ in range(config.num_adapter_layers))
        self.layerdrop = config.layerdrop