Fix precommit check after moving to ruff (#927)

Lint check in main branch is failing. This fixes the lint check after we
moved to ruff in https://github.com/meta-llama/llama-stack/pull/921. We
need to move to a `ruff.toml` file as well as fixing and ignoring some
additional checks.

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

llama_stack/providers/tests/inference/utils.py

@@ -10,7 +10,5 @@ import itertools
 def group_chunks(response):
     return {
         event_type: list(group)
-        for event_type, group in itertools.groupby(
-            response, key=lambda chunk: chunk.event.event_type
-        )
+        for event_type, group in itertools.groupby(response, key=lambda chunk: chunk.event.event_type)
     }