Move abs, frac, reciprocal, and neg to TensorIterator (#19041)
Summary:
I've been messing around with vectorizing the fusion compiler in JIT, and noticed that these ops were pathologically slow. I moved them to use TensorIterator + Vec256<> and got some speed wins.
Benchmark script:
```
import torch, time
ops = ['abs', 'neg', 'reciprocal', 'frac']
x = torch.rand(1024, 1024)
NITER = 10000
print('op', 'time per iter (ms)', 'gops/s', 'GB/s', sep='\t')
for op in ops:
s = time.time()
for i in range(NITER):
getattr(x, op)()
elapsed_sec = ((time.time() - s) / NITER)
print(op, elapsed_sec * 1000, (1024*1024/elapsed_sec)/1e9, (1024*1024*4*2) / elapsed_sec / 1e9, sep='\t')
```
Before this change (on my mac with a skylake):
```
op time per iter (ms) gops/s GB/s
abs 0.9730974197387695 1.0775652866097343 8.620522292877874
neg 1.0723679780960083 0.9778136063534356 7.822508850827485
reciprocal 1.2610594034194946 0.8315040490215421 6.6520323921723366
frac 1.1681334018707275 0.8976509004200546 7.181207203360437
```
After this change:
```
op time per iter (ms) gops/s GB/s
abs 0.5031076192855835 2.084198210889721 16.673585687117768
neg 0.4433974027633667 2.3648672578256087 18.91893806260487
reciprocal 0.47145988941192624 2.2241043693195985 17.79283495455679
frac 0.5036592721939087 2.0819154096627024 16.65532327730162
```
So, after this change it looks like we are hitting machine peak for bandwidth and are bandwidth bound.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19041
Differential Revision: D14862037
Pulled By: jamesr66a
fbshipit-source-id: e2032ac0ca962dbf4120bb36812277c260e22912
