Fix openapi schema (#1947)

Generating the OpenAPI schema was broken under the circumstances:

- train.py#train had an enum-type input
- cog was running with --x-mode=train

I've verified that this change fixes it.  The current code is obviously wrong in
any case.

python/cog/server/http.py

@@ -196,7 +196,7 @@ def train(
 
             @app.put(
                 "/trainings/{training_id}",
-                response_model=PredictionResponse,
+                response_model=TrainingResponse,
                 response_model_exclude_unset=True,
             )
             def train_idempotent(