fix torch.var backward when n==correction (#94546)
Fixes #94184
This PR, as discussed in [comment ](https://github.com/pytorch/pytorch/issues/94184#issuecomment-1422128166), returns `x.grad` of same shape as `x`, and filled with `NaN` when the gradient of `torch.var(unbiased=True)` is `NaN`. The gradient of unbiased variance is `NaN` (undefined, divide by zero in the denom `N-1`, where `N` is the number of samples) when `N` is 1 (i.e., there's one sample only -- product of dim is 1 such as `[1]`, `[1,...,1]`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94546
Approved by: https://github.com/soulitzer
