[ci] Onnx test 3->2 shards (pytorch#97383)

Nit, not entirely sure why onnx needs an extra shard, it also doesn't seem to be doing anything.
https://github.com/pytorch/pytorch/actions/runs/4494513193/jobs/7907327958
The test step is 2 minutes long
Pull Request resolved: pytorch#97383
Approved by: https://github.com/huydhn

.github/workflows/pull.yml

@@ -105,9 +105,8 @@ jobs:
       docker-image-name: pytorch-linux-focal-py3-clang10-onnx
       test-matrix: |
         { include: [
-          { config: "default", shard: 1, num_shards: 3, runner: "linux.2xlarge" },
-          { config: "default", shard: 2, num_shards: 3, runner: "linux.2xlarge" },
-          { config: "default", shard: 3, num_shards: 3, runner: "linux.2xlarge" },
+          { config: "default", shard: 1, num_shards: 2, runner: "linux.2xlarge" },
+          { config: "default", shard: 2, num_shards: 2, runner: "linux.2xlarge" },
         ]}
 
   linux-focal-py3_8-clang10-onnx-test: