Create a dummy batch to go through `m` with `size`.

def dummy_batch(m: nn.Module, size:tuple=(64,64))->Tensor:
    "Create a dummy batch to go through `m` with `size`."
    ch_in = in_channels(m)
    return one_param(m).new(1, ch_in, *size).requires_grad_(False).uniform_(-1.,1.)