[export] Lift constant tensors as buffers (#108592)
When we retrace the graph containing constant tensors, they get lifted as buffer inputs.
AotInductor also wants to lift all the constants as inputs.
If we separate the constants as a separate thing, then it adds an additional complexity where we now have to keep track of 3 inputs (params, buffers, constants).
Cons: People might care about specifically what buffers are/are not buffers?
If people want to know specifically which buffers are constants, we can add an additional field in the graph signature to mark this.
Differential Revision: [D49017872](https://our.internmc.facebook.com/intern/diff/D49017872)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108592
Approved by: https://github.com/zhxchen17
