[1/3] Add header file for Graph in nativert (pytorch#154530)

We split the large PR for adding Graph.h and Graph.cpp to `nativert` into 3 smaller PRs:
1. **Add header file**
2. Add source file
3. Add test and build rules.

Torch Native Runtime RFC: pytorch/rfcs#72

4 classes have been introduced: `Graph`, `Node`, `Value`, `Type`
- `Type` represents the kind of a `Value`
- `Value` represents a single symbolic value, it could be any kind that exists in `Type`. Values are inputs and outputs of a `Node`.
- `Node` represents a single unit of execution, typically a PyTorch op.
- `Graph` represents a model's computation graph, which is designed to facilitate transformation/analysis.

Differential Revision: [D75491860](https://our.internmc.facebook.com/intern/diff/D75491860/)
Pull Request resolved: pytorch#154530
Approved by: https://github.com/SherlockNoMad

Author: yiming0416