Improve unique CPU performance for returning counts (#19352)

Improve unique CPU performance for returning counts (#19352)

Summary:
Benchmark on a tensor of shape `torch.Size([15320, 2])`. Benchmark code:

```python
print(torch.__version__)
print()
a = tensor.flatten()
print('cpu, sorted=False:')
%timeit torch._unique2_temporary_will_remove_soon(a, sorted=False)
%timeit torch._unique2_temporary_will_remove_soon(a, sorted=False, return_inverse=True)
%timeit torch._unique2_temporary_will_remove_soon(a, sorted=False, return_counts=True)
%timeit torch._unique2_temporary_will_remove_soon(a, sorted=False, return_inverse=True, return_counts=True)
print()
print('cpu, sorted=True:')
%timeit torch._unique2_temporary_will_remove_soon(a)
%timeit torch._unique2_temporary_will_remove_soon(a, return_inverse=True)
%timeit torch._unique2_temporary_will_remove_soon(a, return_counts=True)
%timeit torch._unique2_temporary_will_remove_soon(a, return_inverse=True, return_counts=True)
print()
```

Before
```
1.1.0a0+36854fe

cpu, sorted=False:
340 µs ± 4.05 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
724 µs ± 6.28 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
54.3 ms ± 469 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
54.6 ms ± 659 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

cpu, sorted=True:
341 µs ± 7.21 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
727 µs ± 7.05 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
54.7 ms ± 795 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
54.3 ms ± 647 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
```

After
```
1.1.0a0+261d9e8

cpu, sorted=False:
350 µs ± 865 ns per loop (mean ± std. dev. of 7 runs, 1000 loops each)
771 µs ± 598 ns per loop (mean ± std. dev. of 7 runs, 1000 loops each)
1.09 ms ± 6.86 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
1.09 ms ± 4.74 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

cpu, sorted=True:
324 µs ± 4.99 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
705 µs ± 3.18 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
1.09 ms ± 5.22 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
1.09 ms ± 5.63 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19352

Differential Revision: D14984717

Pulled By: VitalyFedyunin

fbshipit-source-id: 3c56f85705ab13a92ec7406f4f30be77226a3210