Constant values are inlined directly into the code of the module, so we currently have no way of accessing them once compilation has run, and the modules in nn.Modules store all their values as constants. I opened this issue 1 so you can track that for fixes.
As a temporary workaround, you could grab the values you care about in Python and stash them as attributes instead of constants:
class M(nn.Module):
    def __init__(self):
        super().__init__()
        self.alex_model = torchvision.models.AlexNet(num_classes=10)
        self.first_conv_stride = getattr(self.alex_model.features, '0').stride

    def forward(self, x):
        return self.alex_model(x)


sm = torch.jit.script(M())
print(sm.first_conv_stride)

and in C++
auto module = torch::jit::load("alexnet.pt");
std::cout << module.attr("first_conv_stride") << "\n";