bugfix generic-k code in top-k with softmax

[t4c1]

Fixes a bug in generic top-k softmax EVT implementation that resulted in wrong results when k != 2 and k != 4.

This allows removal of the static assert requiring k to be either 2 or 4. Comment in example 61 is also fixed to reflect that any k value is now supported.

[alihassanijr]

@t4c1 Thank you for submitting this patch.

Just a note on the assertion, it's there more as a warning to users that the generic sort comes with serious performance implications. Due to the control flow associated with it, it introduces somewhat heavy branching and register spilling, resulting in a smaller improvement over the baseline.

I'll let @hwu36 chime in on whether we want to get rid of the assertion.

[hwu36]

@alihassanijr , maybe leave it there but make the text clearer about the consequences?

[alihassanijr]

@hwu36 leave the assert there with a better message, or remove and make the doc clearer about the consequences?

[hwu36]

Leave it there and make the message better so people willing to try can try it.

[t4c1]

I addressed the review comments some time ago. Is something else required to get this merged?