add get buffer from exported program (#107809)
Summary: We have the util function to get params, for parity we also need util function to get buffer`
Test Plan:
```
buck test //caffe2/test:test_export
```
Differential Revision: D48610877
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107809
Approved by: https://github.com/JacobSzwejbka
