Fix slogdet sign requiring grad when input requires grad (#16337)
Summary:
The real fix for https://github.com/pytorch/pytorch/issues/15605.
This is sort of BC breaking because now
```py
In [1]: import torch
In [2]: a = torch.randn(3, 3, requires_grad=True)
In [3]: a.slogdet()
Out[3]: (tensor(1.), tensor(0.1356, grad_fn=<SlogdetBackward>))
In [4]: a.slogdet()[0].requires_grad
Out[4]: False
```
while before this patch ` a.slogdet()[0]` requires grad with `grad_fn=<SlogdetBackward>`. But any use of backproping through this value will meet the error in #15605 so I don't think this is a problem.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16337
Differential Revision: D13832644
Pulled By: soumith
fbshipit-source-id: f96c477e99edcbdbd966888e5c5ea7fd058429a8
