Move abs, frac, reciprocal, and neg to TensorIterator (#19041)

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