server: add logprobs and top_logprobs support to Ollama's API (#12899)

Adds logprobs support to Ollama's API including support for Ollama's
OpenAI-compatible API. By specifying the new 'logprobs' boolean parameter
in the API, Ollama will return the log probabilities for each token generated.
'top_logprobs', an integer value can also be specified up to the value 20.
When specified, the API will also provide the number of most likely tokens to
return at each token position

Co-authored-by: Baptiste Jamin <baptiste@crisp.chat>

llm/server.go

@@ -1362,6 +1362,12 @@ type CompletionRequest struct {
 	Grammar  string // set before sending the request to the subprocess
 	Shift    bool
 	Truncate bool
+
+	// Logprobs specifies whether to include log probabilities in the response
+	Logprobs bool
+
+	// TopLogprobs specifies the number of most likely alternative tokens to return (0-20)
+	TopLogprobs int
 }
 
 // DoneReason represents the reason why a completion response is done
@@ -1387,6 +1393,18 @@ func (d DoneReason) String() string {
 	}
 }
 
+// TokenLogprob represents log probability information for a single token alternative.
+type TokenLogprob struct {
+	Token   string  `json:"token"`
+	Logprob float64 `json:"logprob"`
+}
+
+// Logprob contains log probability information for a generated token.
+type Logprob struct {
+	TokenLogprob
+	TopLogprobs []TokenLogprob `json:"top_logprobs,omitempty"`
+}
+
 type CompletionResponse struct {
 	Content            string        `json:"content"`
 	DoneReason         DoneReason    `json:"done_reason"`
@@ -1395,6 +1413,9 @@ type CompletionResponse struct {
 	PromptEvalDuration time.Duration `json:"prompt_eval_duration"`
 	EvalCount          int           `json:"eval_count"`
 	EvalDuration       time.Duration `json:"eval_duration"`
+
+	// Logprobs contains log probability information if requested
+	Logprobs []Logprob `json:"logprobs,omitempty"`
 }
 
 func (s *llmServer) Completion(ctx context.Context, req CompletionRequest, fn func(CompletionResponse)) error {
@@ -1530,7 +1551,8 @@ func (s *llmServer) Completion(ctx context.Context, req CompletionRequest, fn fu
 
 			if c.Content != "" {
 				fn(CompletionResponse{
-					Content: c.Content,
+					Content:  c.Content,
+					Logprobs: c.Logprobs,
 				})
 			}