Migrate `exp` and `exp_` from the TH to Aten (CUDA) (#36652)
Summary:
Closes https://github.com/pytorch/pytorch/issues/24561
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.exp(a) a.numel() == {n} for {t} times {dtype}')
print(timeit.timeit(f'torch.exp(a); torch.cuda.synchronize()',
setup=f'import torch; a=torch.arange({n}, dtype={dtype}, device="cuda")',
number=t))
```
Before:
```
torch.exp(a) a.numel() == 10000 for 20000 times torch.half
0.3001665159999902
torch.exp(a) a.numel() == 10000 for 20000 times torch.float
0.28265794499998265
torch.exp(a) a.numel() == 10000 for 20000 times torch.double
0.3432170909998149
torch.exp(a) a.numel() == 100000 for 20000 times torch.half
0.32273333800003456
torch.exp(a) a.numel() == 100000 for 20000 times torch.float
0.31498759600003723
torch.exp(a) a.numel() == 100000 for 20000 times torch.double
1.079708754999956
```
After:
```
torch.exp(a) a.numel() == 10000 for 20000 times torch.half
0.27996097300092515
torch.exp(a) a.numel() == 10000 for 20000 times torch.float
0.2774473429999489
torch.exp(a) a.numel() == 10000 for 20000 times torch.double
0.33066844799941464
torch.exp(a) a.numel() == 100000 for 20000 times torch.half
0.27641824200145493
torch.exp(a) a.numel() == 100000 for 20000 times torch.float
0.27805968599932385
torch.exp(a) a.numel() == 100000 for 20000 times torch.double
1.0644143180015817
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36652
Differential Revision: D21164653
Pulled By: VitalyFedyunin
fbshipit-source-id: 42c7b24b0d85ff1d390231f1457968a8869b8db3
