[nnc] Add a default constructor for Placeholder
Summary:
It's useful to be able to have an uninitialized Placeholder,
sometimes, e.g., as a class member, where member initialization is
awkward/impossible.
(Yes, one could wrap a Placeholder in a unique_ptr, but it's an extra layer of
cruft).
Test Plan: `buck build //caffe2/test:jit`
Reviewed By: navahgar
Differential Revision: D27400784
fbshipit-source-id: 56191ee11cbb4bc91b5624af6329f2d6d007570b
