[pt2 export] small fix on non_persistent buffer unlift (#120715)
Summary: Change to get_buffer from the input plain_graph_module instead of the new stateful_gm when restoring non_persistent buffers, since the stateful_gm doesn't contain the buffer yet.
Test Plan:
Added test case.
`buck test caffe2/test:test_export -- test_unlift_nonpersistent_buffer`
Differential Revision: D54216772
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120715
Approved by: https://github.com/zhxchen17
