Migrate `var` & `std` to ATen (#39967)
Summary:
Not sure why there are so many issues for std & var, but this PR should close them all:
std: Fix https://github.com/pytorch/pytorch/issues/24771, Fix https://github.com/pytorch/pytorch/issues/24676, Fix https://github.com/pytorch/pytorch/issues/24639, Fix https://github.com/pytorch/pytorch/issues/24529
var: Fix https://github.com/pytorch/pytorch/issues/24782, Fix https://github.com/pytorch/pytorch/issues/24677, Fix https://github.com/pytorch/pytorch/issues/24652, Fix https://github.com/pytorch/pytorch/issues/24530
```py
import time
import torch
def _time():
if torch.cuda.is_available():
torch.cuda.synchronize()
return time.time()
for device in (torch.device("cpu"), torch.device("cuda")):
for size in (
[100000000],
[10000, 10000],
[1000, 1000, 100],
[100, 100, 100, 100],
):
t = torch.randn(*size, device=device)
total_time = 0
for i in range(10):
t1 = _time()
t.std()
t2 = _time()
total_time += t2 - t1
print(f"Tensor of size {size} on {device}: {total_time / 10}")
```
Before:
```
Tensor of size [100000000] on cpu: 0.36041643619537356
Tensor of size [10000, 10000] on cpu: 0.37235140800476074
Tensor of size [1000, 1000, 100] on cpu: 0.386572527885437
Tensor of size [100, 100, 100, 100] on cpu: 0.37404844760894773
Tensor of size [100000000] on cuda: 0.0021645784378051757
Tensor of size [10000, 10000] on cuda: 0.002090191841125488
Tensor of size [1000, 1000, 100] on cuda: 0.00208127498626709
Tensor of size [100, 100, 100, 100] on cuda: 0.0020844221115112306
```
After:
```
Tensor of size [100000000] on cpu: 0.1339871883392334
Tensor of size [10000, 10000] on cpu: 0.1343991994857788
Tensor of size [1000, 1000, 100] on cpu: 0.1346735954284668
Tensor of size [100, 100, 100, 100] on cpu: 0.11906447410583496
Tensor of size [100000000] on cuda: 0.0013531208038330077
Tensor of size [10000, 10000] on cuda: 0.0012922048568725585
Tensor of size [1000, 1000, 100] on cuda: 0.001285886764526367
Tensor of size [100, 100, 100, 100] on cuda: 0.0012899160385131836
```
cc: VitalyFedyunin
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39967
Differential Revision: D22162469
Pulled By: VitalyFedyunin
fbshipit-source-id: 8d901c779767b00f81cd6231bc665e04f297b4c3
