if I understand you correctly, you can build a new model such as follows
class SampleModel(nn.Module):
    def __init__(self, num_channel_in=16*3):
        super(SampleModel, self).__init__()
        self.c1 = nn.Conv2d(num_channel_in, 128, kerrnel_size=3, stride=1, padding=1)
        self.c2 = nn.Conv2d(128, 256, kerrnel_size=3, stride=1, padding=1)
        ...