Fix build error in VariableTypeManual
Summary:
build error in internal pt mobile build
```
xplat/caffe2/torch/csrc/autograd/VariableTypeManual.cpp:118:49: error: address of function 'requires_grad' will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]
autograd::utils::requires_grad_leaf_error(requires_grad)
~~~~~~~~ ^~~~~~~~~~~~~
xplat/caffe2/torch/csrc/autograd/VariableTypeManual.cpp:118:49: note: prefix with the address-of operator to silence this warning
```
I think the variable name in requires_grad_leaf_error is wrong.
Test Plan: mobile build works
Reviewed By: pbelevich
Differential Revision: D18192663
fbshipit-source-id: a3d3ebb9039022eb228c1d183a1076f65f9e84e0
