def _calc_channel(start_c, num_blk):
    """
    Calculate the number of in and out channels of each stages in FishNet.
    Example:
        fish150 : start channel=64, num_blk=3,
        tail channels : Grow double in each stages,
                        [64, 128, 256 ...] = [start channel ** (2**num_blk) ....]
        body channels : In first stage, in_channel and out_channel is the same,
                        but the other layers, the number of output channels is half of the number of input channel
                        Add the number of transfer channels to the number of output channels
                        The numbers of transfer channels are reverse of the tail channel[:-2]
                        [(512, 512), + 256
                         (768, 384), + 128
                         (512, 256)] + 64
        head channels : The number of input channels and output channels is the same.
                        Add the number of transfer channels to the number of output channels
                        The numbers of transfer channels are reverse of the tail channel[:-2]
                        [(320, 320),   + 512
                         (832, 832),   + 768
                         (1600, 1600)] + 512
    """
    # tail channels
    tail_channels = [start_c]
    for i in range(num_blk):
        tail_channels.append(tail_channels[-1] * 2)
    print("Tail Channels : ", tail_channels)

    # body channels
    in_c, transfer_c = tail_channels[-1], tail_channels[-2]
    body_channels = [(in_c, in_c), (in_c + transfer_c, (in_c + transfer_c) // 2)]
    for i in range(1, num_blk - 1):
        transfer_c = tail_channels[-i - 2]
        in_c = body_channels[-1][1] + transfer_c
        body_channels.append((in_c, in_c // 2))
    print("Body Channels : ", body_channels)

    # head channels
    in_c = body_channels[-1][1] + tail_channels[0]
    head_channels = [(in_c, in_c)]
    for i in range(num_blk):
        transfer_c = body_channels[-i - 1][0]
        in_c = head_channels[-1][1] + transfer_c
        head_channels.append((in_c, in_c))
    print("Head Channels : ", head_channels)
    return {"tail_channels": tail_channels, "body_channels": body_channels, "head_channels": head_channels}