Use Module `__getattr__` in `torcharrow.functional`

[wenleix]

Follows torch.ops, torcharrow.functional creates a runtime module instead of a "physical" module.

The reason is torch.ops wants to override __getattr__ to support lazy loading customized ops, which is only supported for runtime module before Python 3.7

With Python 3.7, Module __getattr__ can is overridable natively. PyTorch plans to use module __getattr__ in torch.ops: pytorch/pytorch#72448.. torcharrow.functional should follow this :)