improve performance of unique with inverse indices (#16145)

improve performance of unique with inverse indices (#16145)

Summary:
Partial fix for #15804, only w/o dim.
For jcjohnson benchmarking script I'm getting the following results on V100:
Before:
```
unning with N = 10000, M = 10000
cuda (no inverse): 0.98 ms
cpu (no inverse): 0.96 ms
cuda (with inverse): 1.07 ms
cpu (with inverse): 1.76 ms

Running with N = 10000, M = 100000
cuda (no inverse): 0.76 ms
cpu (no inverse): 1.53 ms
cuda (with inverse): 1.23 ms
cpu (with inverse): 3.02 ms

Running with N = 100000, M = 100000
cuda (no inverse): 1.28 ms
cpu (no inverse): 11.22 ms
cuda (with inverse): 69.76 ms
cpu (with inverse): 20.28 ms

Running with N = 100000, M = 1000000
cuda (no inverse): 0.78 ms
cpu (no inverse): 18.78 ms
cuda (with inverse): 133.45 ms
cpu (with inverse): 34.09 ms

Running with N = 500000, M = 500000
cuda (no inverse): 1.43 ms
cpu (no inverse): 61.13 ms
cuda (with inverse): 3315.18 ms
cpu (with inverse): 104.57 ms

Running with N = 500000, M = 5000000
cuda (no inverse): 0.86 ms
cpu (no inverse): 96.44 ms
cuda (with inverse): 5209.93 ms
cpu (with inverse): 176.10 ms
```
After
```
Running with N = 10000, M = 10000
cuda (no inverse): 1.04 ms
cpu (no inverse): 0.94 ms
cuda (with inverse): 0.64 ms
cpu (with inverse): 1.76 ms

Running with N = 10000, M = 100000
cuda (no inverse): 0.77 ms
cpu (no inverse): 1.55 ms
cuda (with inverse): 0.58 ms
cpu (with inverse): 2.79 ms

Running with N = 100000, M = 100000
cuda (no inverse): 1.30 ms
cpu (no inverse): 14.15 ms
cuda (with inverse): 1.63 ms
cpu (with inverse): 20.90 ms

Running with N = 100000, M = 1000000
cuda (no inverse): 0.82 ms
cpu (no inverse): 18.63 ms
cuda (with inverse): 0.61 ms
cpu (with inverse): 33.52 ms

Running with N = 500000, M = 500000
cuda (no inverse): 1.51 ms
cpu (no inverse): 59.81 ms
cuda (with inverse): 1.23 ms
cpu (with inverse): 110.69 ms

Running with N = 500000, M = 5000000
cuda (no inverse): 0.92 ms
cpu (no inverse): 104.26 ms
cuda (with inverse): 0.84 ms
cpu (with inverse): 187.12 ms
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16145

Differential Revision: D13738821

Pulled By: soumith

fbshipit-source-id: 0811fb4ade47e3b466cebbc124e3f3333a986749