[Relax][PyTorch] Enhance handling of unbounded upper bound constraints

[guan404ming]

Why

PyTorch uses int_oo (IntInfinity) for unbounded constraints, which would make our current implemenation crash

How

• Update the type hint for create_input_vars to allow for optional upper bounds.
• Modify the logic to handle unbounded constraints by setting upper bounds to None when applicable.
• Add a new test case