[BE] Do not expose `torch.functional.opt_einsum` (#102004)
It's not mentioned in `__all__`, so moving `import torch.backends.opt_einsum
as opt_einsum` into `einsum` function to delay `torch.backends` import and hide it completely from the module scope.
level module.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102004
Approved by: https://github.com/janeyx99
