Fix elu backward operation for negative alpha (#49272)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/47671
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49272
Test Plan:
```
x = torch.tensor([-2, -1, 0, 1, 2], dtype=torch.float32, requires_grad=True)
y = torch.nn.functional.elu_(x.clone(), alpha=-2)
grads = torch.tensor(torch.ones_like(y))
y.backward(grads)
```
```
RuntimeError: In-place elu backward calculation is triggered with a negative slope which is not supported.
This is caused by calling in-place forward function with a negative slope, please call out-of-place
version instead.
```
Reviewed By: albanD
Differential Revision: D25569839
Pulled By: H-Huang
fbshipit-source-id: e3c6c0c2c810261566c10c0cc184fd81b280c650
