I found boxes is not tensor, it’s Optional[Tensor]. It’s inferred as Optional[Tensor] because of the control flow in the east model. The forward method of model is like this:
def forward(self,x):
    if condition1:
        return None
    else:
        return boxes

It’s inferred as Optional[Tensor] because the return value of forward can be None or tensor.
And size() method if not supported by Optional[Tensor].