PyTorch's ParsedArgs::tensor() copies the returned tensor

e.g.: pytorch/torch/csrc/autograd/python_variable.cpp
```c++
static PyObject* THPVariable_make_subclass(PyObject* _ignored, PyObject* args, PyObject* kwargs) {
...
  auto data = r.tensor(1).detach();
```

That `r.tensor(1)` copies the tensor and then detaches. Unnecessary copy.

`PythonArgs::optionalTensor` should be changed to return a null/non-null ptr instead of optional<Tensor>