fix specify_constraints's signature when exporting model (#100739)
Currently, when f is a Module, the signature should be the "forward" methods signature. For example,
```python
class Module(torch.nn.Module):
def forward(self, x):
return x.sin()
mod = Module()
x = torch.ones([3, 3])
torch._dynamo.export(mod, x, constraints=[dynamic_dim(x, 0)])
```
Previously, it prints following:
```python
def specify_constraints(*args, **kwargs):
return [
2 <= dynamic_dim(x, 0),
2 <= dynamic_dim(x, 1),
]
```
After the pr, it prints:
```python
def specify_constraints(x):
return [
2 <= dynamic_dim(x, 0),
2 <= dynamic_dim(x, 1),
]
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100739
Approved by: https://github.com/avikchaudhuri
