chore: add request validation and better error message for n > 1 and temperature=0 (#3914)

Signed-off-by: zhongdaor <zhongdaor@nvidia.com>

lib/llm/src/protocols/openai/chat_completions.rs

@@ -305,6 +305,8 @@ impl ValidateRequest for NvCreateChatCompletionRequest {
         validate::validate_repetition_penalty(self.get_repetition_penalty())?;
         validate::validate_min_p(self.get_min_p())?;
         validate::validate_top_k(self.get_top_k())?;
+        // Cross-field validation
+        validate::validate_n_with_temperature(self.inner.n, self.inner.temperature)?;
 
         Ok(())
     }

lib/llm/src/protocols/openai/completions.rs

@@ -396,6 +396,8 @@ impl ValidateRequest for NvCreateCompletionRequest {
         validate::validate_repetition_penalty(self.get_repetition_penalty())?;
         validate::validate_min_p(self.get_min_p())?;
         validate::validate_top_k(self.get_top_k())?;
+        // Cross-field validation
+        validate::validate_n_with_temperature(self.inner.n, self.inner.temperature)?;
 
         Ok(())
     }

lib/llm/src/protocols/openai/validate.rs

@@ -249,6 +249,27 @@ pub fn validate_n(n: Option<u8>) -> Result<(), anyhow::Error> {
     Ok(())
 }
 
+/// Validates n and temperature interaction
+/// When n > 1, temperature must be > 0 to ensure diverse outputs
+pub fn validate_n_with_temperature(
+    n: Option<u8>,
+    temperature: Option<f32>,
+) -> Result<(), anyhow::Error> {
+    if let Some(n_value) = n
+        && n_value > 1
+    {
+        let temp = temperature.unwrap_or(1.0);
+        if temp == 0.0 {
+            anyhow::bail!(
+                "When n > 1, temperature must be greater than 0 to ensure diverse outputs. Got n={}, temperature={}",
+                n_value,
+                temp
+            );
+        }
+    }
+    Ok(())
+}
+
 /// Validates model parameter
 pub fn validate_model(model: &str) -> Result<(), anyhow::Error> {
     if model.trim().is_empty() {