Make test_torch_predictor a medium test. (ray-project#35466)

Looking at test_torch_predictor, it is a suite of 35 test cases, and runs roughly a minute of time.
Most of the time these tests do finish fine actually. That's why it's just flaky.
Make this a medium test to de-flake it.

Signed-off-by: Jun Gong <jungong@anyscale.com>

python/ray/train/BUILD

@@ -498,7 +498,7 @@ py_test(
 
 py_test(
     name = "test_torch_predictor",
-    size = "small",
+    size = "medium",
     srcs = ["tests/test_torch_predictor.py"],
     tags = ["team:ml", "exclusive", "ray_air", "gpu"],
     deps = [":train_lib", ":conftest"]