Fix graph signature data model to list of specs. (#836)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/836

X-link: https://github.com/pytorch/pytorch/pull/111017

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,
),
...
]
```

Reviewed By: angelayi

Differential Revision: D49876258

fbshipit-source-id: f5612d47c3a6631a3898395c42e694ed57037085