tfb_softplus(hinge_softness = NULL, validate_args = FALSE, name = "softplus")

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

For large x >> 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 > 0 approaches 0 from the right, f_c(x) becomes less and less soft,
approaching max(0, x).
* c = 1 is the default.
* c > 0 but small means f(x) approx ReLu(x) = max(0, x).
* c < 0 flips sign and reflects around the y-axis: f_{-c}(x) = -f_c(-x).
* c = 0 results in a non-bijective transformation and triggers an exception.
Note: log(.) and exp(.) are applied element-wise but the Jacobian is a reduction over the event space.

[1 + exp(x / c)]: R:1%20+%20exp(x%20/%20c)
[1 + exp(x / c)]: R:1%20+%20exp(x%20/%20c)
[exp(x / c)]: R:exp(x%20/%20c)