feat(tests): replace manual sp_group retrieval with module attribute

- Use `sequence_parallel._sp_group` directly instead of calling `_get_sp_group_from_device_mesh`
- Simplifies test setup by relying on internal module state after `_setup_sp`

Author: meichangsu1

tests/sequence_parallel/test_sequence_parallel_single_attention.py

@@ -181,7 +181,7 @@ def _run_worker_single_attn(rank: int, world_size: int, port: int, padding: bool
         sp_size = world_size
         device_mesh = DeviceMesh.from_sizes(dp_size=world_size, ulysses_size=sp_size, device_type="cuda")
         _setup_sp(device_mesh, sp_size)
-        sp_group = _get_sp_group_from_device_mesh(device_mesh, sp_size)
+        sp_group = sequence_parallel._sp_group
 
         batch_size = 2
         unpad_seq_len = 127 if padding else 128
@@ -271,7 +271,7 @@ def _run_worker_single_attn_fsdp(rank: int, world_size: int, port: int):
         # For FSDP+SP, SP is derived from dp/fsdp ranks. Use fsdp=world, dp=1.
         device_mesh = DeviceMesh.from_sizes(fsdp_size=world_size, dp_size=1, ulysses_size=sp_size, device_type="cuda")
         _setup_sp(device_mesh, sp_size)
-        sp_group = _get_sp_group_from_device_mesh(device_mesh, sp_size)
+        sp_group = sequence_parallel._sp_group
 
         batch_size = 2
         unpad_seq_len = 128