Added missing namespaces for C++ API (#45736)
Summary:
Hello,
depending on the build environment you may encounter
```c++
error: reference to 'optional' is ambiguous
```
when using the Torch-C++-API.
This PR adds `c10::` to avoid possible ambiguities with **std::optional** and does not introduce any functional change.
Fixes https://discuss.pytorch.org/t/linker-failed-with-ambiguous-references/36255 .
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45736
Reviewed By: dzhulgakov
Differential Revision: D24125123
Pulled By: VitalyFedyunin
fbshipit-source-id: df21420f0a2d0270227c28976a7a4218315cc107
