[export] allow Dim(1,2) for export dynamic shapes (#121642)
Current dynamic shapes implementation fixes lower range of Dims to be 2 for analysis, but allows 0/1 shapes during runtime. This leads to failures when initializing Dim(1,2). This PR sets the lower bound to 0, and avoids erroring out when conflicting with the generated (2, maxsize) constraint during analysis.
Also resolves a derived dim constraints issue with the following code:
```
class Bar(torch.nn.Module):
def forward(self, x, y):
return x + y[1:]
dx = Dim("dx", min=1, max=3)
ep = export(
Bar(),
(torch.randn(2, 2), torch.randn(3, 2)),
dynamic_shapes=({0: dx, 1: None}, {0: dx+1, 1: None})
)
print(ep.range_constraints)
```
In main:
```
{s0: ValueRanges(lower=2, upper=3, is_bool=False), s0 + 1: ValueRanges(lower=3, upper=4, is_bool=False)}
```
This PR:
```
{s0: ValueRanges(lower=1, upper=3, is_bool=False), s0 + 1: ValueRanges(lower=2, upper=4, is_bool=False)}
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121642
Approved by: https://github.com/avikchaudhuri
