log_softmax (#3411)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3411

Use the following property to implement `log_softmax`
```
log(exp(x_i) / sum(exp(x_j))) = x_i - log(sum(exp(x_j)))
```
- numerically stable, since we can avoid log(small_number) -> -\infty
- reusing existing `softmax` shader to generate variants

Reviewed By: yipjustin, SS-JIA

Differential Revision: D56688093

fbshipit-source-id: bfc58103a0f09e305f6bf47e764f6d44519f75d4