[2/3][lite interpreter] add metadata when saving and loading models for mobile
Summary:
1. add `metadata.pkl` to `.bc` file which includes the model info that we are interested in
2. load `metadata.pkl` as a attribute `unordered_map<string, string>` in the module
Test Plan:
- CI
```buck build //xplat/caffe2:jit_module_saving
```
```buck build //xplat/caffe2:torch_mobile_core
```
Reviewed By: xcheng16
Differential Revision: D23047144
fbshipit-source-id: f368d00f7baef2d3d15f89473cdb146467aa1e0b
