[export] Fix graph signature data model to list of specs. (#111017)
Summary:
Previously we design the GraphSignature format as a bunch of inputs and outputs node names. After a discussion in the design meeting we decide to change the format to make signature more self-contained. Now the signature format look like the following:
```
[
InputSpec(
kind=InputKind.USER_INPUT,
arg=TensorArgument(name="arg0_1"),
target=None,
),
...
]
```
Test Plan: CI
Reviewed By: angelayi
Differential Revision: D49876258
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111017
Approved by: https://github.com/angelayi
