fixed return type for cuda.memory.mem_get_info() (#81073)
Return type was `int` but function actually returns a tuple of two ints. The first being the free gpu memory in bytes and the second being the total available gpu memory in bytes.
Return type was fixed to correctly read `Tuple[int, int]` and the `Tuple` class was imported from `typing`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81073
Approved by: https://github.com/ngimel
