Add temperature sanity check as 0 is invalid (#1211)

A user was passing 0 and this results in a divide by zero in the SoftMax
calculation, which results in +INF/NaN floating point results that are
not obviously caused by a zero temperature.

This lets users know at token selection time that the current
temperature is invalid.

src/generators.cpp

@@ -446,6 +446,8 @@ void Generator::GenerateNextToken() {
     throw std::runtime_error("top_p must be between 0.0 and 1.0");
   if (search.top_k < 0)
     throw std::runtime_error("top_k must be 0 or greater");
+  if (search.temperature <= 0.0f)
+    throw std::runtime_error("temperature must be greater than 0");
 
   if (search.top_p > 0.0f && search.top_p < 1.0f && search.top_k > 1) {
     search_->SampleTopKTopP(search.top_k, search.top_p, search.temperature);