model: Don't unconditionally add special tokens

We sometimes tokenize partial strings. For example, with
multimodal inputs, we split the input string around the images
and then tokenize each piece. In these cases, we should only add
the special tokens on the first piece.

runner/ollamarunner/runner.go

@@ -161,7 +161,7 @@ func (s *Server) inputs(prompt string, images []ImageData) ([]input, error) {
 
 	for i, part := range parts {
 		// text - tokenize
-		tokens, err := s.model.(model.TextProcessor).Encode(part)
+		tokens, err := s.model.(model.TextProcessor).Encode(part, i == 0)
 		if err != nil {
 			return nil, err
 		}