onnxruntime

Implement a more stable softmax (#2715)

* Implement a more stable SoftMax
 e^x is represented as infinity if x is large enough, like 100.f. Infinity divided by Infinity is a NAN. Thus, softmax gets a NAN if one or more item are large enough.
A math transform as below is leveraged to get a stable softmax:
e^xi/(e^x1 + ...e^xn) = e^(xi - max) / (e^(x1 - max) + ... + e^(xn - max))

And for convenience, force max to 0.f if all xi are negative