g(x / c) = c * Log[1 + exp(x / c)].
```

For large x &gt;&gt; 1,

```
c * Log[1 + exp(x / c)] approx c * Log[exp(x / c)] = x
```

so the behavior for large x is the same as the standard softplus.
As c &gt; 0 approaches 0 from the right, f_c(x) becomes less and less soft,
approaching max(0, x).
* c = 1 is the default.
* c &gt; 0 but small means f(x) approx ReLu(x) = max(0, x).
* c &lt; 0 flips sign and reflects around the y-axis: f_{-c}(x) = -f_c(-x).
* c = 0 results in a non-bijective transformation and tri