def find_layer(arch, target_layer_name):
    """Find target layer to calculate CAM.

        : Args:
            - **arch - **: Self-defined architecture.
            - **target_layer_name - ** (str): Name of target class.

        : Return:
            - **target_layer - **: Found layer. This layer will be hooked to get forward/backward pass information.
    """

    if target_layer_name.split('_') not in arch._modules.keys():
        raise Exception("Invalid target layer name.")
    target_layer = arch._modules[target_layer_name]
    return target_layer