Enable torch._refs.var for nvFuser executor (#79517)
This PR adds variance function with correction argument to nvFuser.
Now it's possible to run
```py
import torch
import torch._refs
from torch._prims.executor import make_traced
def foo1(a):
return torch._refs.var(a, keepdim=False, unbiased=False)
def foo2(a):
return torch._refs.var(a, keepdim=False, correction=2)
a = torch.randn(3, 3, device='cuda')
make_traced(foo1)(a, executor="nvfuser")
make_traced(foo2)(a, executor="nvfuser")
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79517
Approved by: https://github.com/mruberry, https://github.com/jjsjann123
