[export] preserve constant fqn (#120664)
Summary:
Previously we were renaming constants to `lifted_constant_tensor0` or equivalent. This PR changes things so that the constants retain the same FQN as in the original eager module.
Actually, `symbolic_trace` already is supposed to do this, but the code path is not triggered when used from `make_fx`, since we don't pass an actual `nn.Module` instance to `trace()`, but rather a multiply-wrapped-functionalized-lambda-thing.
So, I reproduced the essential logic outside of make_fx, at the export layer.
Test Plan: added a unit test
Differential Revision: D54221616
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120664
Approved by: https://github.com/SherlockNoMad
