Migrate `tanh` from TH to ATen (CUDA) (#36995)
Summary:
Closes https://github.com/pytorch/pytorch/issues/24642
Benchmark with same build settings on same system.
gcc : version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)
CUDA : 10.1
GPU : 1050ti
```python
import timeit
for n, t in [(10_000, 20000),
(100_000, 20000)]:
for dtype in ('torch.half', 'torch.float', 'torch.double'):
print(f'torch.tanh(a) a.numel() == {n} for {t} times {dtype}')
print(timeit.timeit(f'torch.tanh(a); torch.cuda.synchronize()',
setup=f'import torch; a=torch.arange({n}, dtype={dtype}, device="cuda")',
number=t))
```
Before:
```
torch.tanh(a) a.numel() == 10000 for 20000 times torch.half
0.2816318240002147
torch.tanh(a) a.numel() == 10000 for 20000 times torch.float
0.2728829070001666
torch.tanh(a) a.numel() == 10000 for 20000 times torch.double
0.39797203200214426
torch.tanh(a) a.numel() == 100000 for 20000 times torch.half
0.3228214350019698
torch.tanh(a) a.numel() == 100000 for 20000 times torch.float
0.31780802399953245
torch.tanh(a) a.numel() == 100000 for 20000 times torch.double
1.3745740449994628
```
After:
```
torch.tanh(a) a.numel() == 10000 for 20000 times torch.half
0.27825374500025646
torch.tanh(a) a.numel() == 10000 for 20000 times torch.float
0.27764024499992956
torch.tanh(a) a.numel() == 10000 for 20000 times torch.double
0.3771585260001302
torch.tanh(a) a.numel() == 100000 for 20000 times torch.half
0.2995866400015075
torch.tanh(a) a.numel() == 100000 for 20000 times torch.float
0.28355561699936516
torch.tanh(a) a.numel() == 100000 for 20000 times torch.double
1.393811182002537
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36995
Differential Revision: D21163353
Pulled By: ngimel
fbshipit-source-id: e2216ff62cdfdd13b6a56daa63d4ef1440d991d4
