Add some info logs about model format checking.

model.ml

@@ -180,6 +180,7 @@ let retrieve_nnet_metadata filename =
 
 let build ~filename =
   let open Result in
+  Logs.info (fun m -> m "Checking format of model file `%s'." filename);
   if Sys.file_exists filename
   then
     begin
@@ -189,6 +190,9 @@ let build ~filename =
         retrieve_nnet_metadata filename >>= fun nnet ->
         Ok (Nnet nnet)
     end >>= fun format ->
+    Logs.info (fun m ->
+        m "Model format recognized as `%s'."
+          (match format with Onnx -> "ONNX" | Nnet _ -> "NNet"));
     Ok { format; filename }
   else
     Error (Format.sprintf "No such file `%s'." filename)