import torch

class FooBar(torch.jit.ScriptModule):
    def __init__(self, my_optional):
        super().__init__()
        self.my_optional = my_optional

    @torch.jit.script_method
    def forward(self, x):
        if self.my_optional is not None:
            return torch.neg(x) + self.my_optional
        return torch.neg(x)

x = torch.rand(3, 4)
fb = FooBar(None)
print(fb(x))

RuntimeError: 
attribute 'my_optional' does not exist:
@torch.jit.script_method
def forward(self, x):
    if self.my_optional is not None:
       ~~~~~~~ <--- HERE
        return torch.neg(x) + self.my_optional
    return torch.neg(x)