onnxruntime

Improve TopK performance. (#3612)

* Update TopK implementation.
  - add faster heap
  - special case k=1
  - update selector for when to use heap and when to use nth_element based on performance testing
  - parallelize if enough work to do
  - reduce templatized code
  - add some extra unit tests.

Perf tested vs. master. Average speedup is 3.75x using this combination of input sizes:

```
    batches = [10, 25, 50]
    batch_size = [8, 16, 32, 64, 128, 256, 512, 1024, 2048]
    k = [1, 2, 4, 6, 8, 16, 24, 32, 48, 64, 128]
```

For larger batches (e.g. 50x2048) the speedup is over 20x.