Revert "[BE] replace the extra DeviceMesh _flatten with mesh access (#666)"

This reverts commit 3653bf2.

Author: XilunWu

torchtitan/parallelisms/parallelize_llama.py

@@ -34,7 +34,6 @@
 from torchtitan.config_manager import JobConfig, TORCH_DTYPE_MAP
 from torchtitan.logging import logger
 from torchtitan.parallelisms.parallel_dims import ParallelDims
-from torchtitan.parallelisms.utils import check_if_feature_in_pytorch
 
 
 def parallelize_llama(
@@ -80,31 +79,22 @@ def parallelize_llama(
     if (
         parallel_dims.dp_shard_enabled
     ):  # apply FSDP or HSDP, potentially with Context Parallel
-        try:
-            dp_mesh = (
-                world_mesh["dp_cp"] if parallel_dims.cp_enabled else world_mesh["dp"]
-            )
-        except IndexError:
-            # note: this is a workaround of the above logic for old pytorch version
-            # where https://github.com/pytorch/pytorch/pull/138945 is not included
-            # throw a warning to encourage users to upgrade to a newer pytorch version
-            check_if_feature_in_pytorch(
-                "DeviceMesh flattening over 3D+ meshes",
-                "https://github.com/pytorch/pytorch/pull/138945",
-                "2.6.0.dev20241030",
-            )
-            # TODO: remove this workaround once PyTorch 2.6 is released
-            dp_mesh_dim_names = (
-                ("dp_replicate", "dp_shard")
-                if parallel_dims.dp_replicate_enabled
-                else ("dp",)
-            )
-            # note that mesh can only be flattened from the finest-grained mesh dimensions
-            dp_mesh = (
-                world_mesh[(*dp_mesh_dim_names, "cp")]._flatten("dp_cp")
-                if parallel_dims.cp_enabled
-                else world_mesh[dp_mesh_dim_names]
-            )
+
+        # TODO: instead of flattening the mesh twice, we could've done in a batter way:
+        # dp_mesh = world_mesh["dp_cp"] if parallel_dims.cp_enabled else world_mesh["dp"]
+        # However, this leads to an error in `DeviceMesh.__get_item__` which I believe is
+        # a bug in DeviceMesh. We should fix it and then use the above line.
+        dp_mesh_dim_names = (
+            ("dp_replicate", "dp_shard")
+            if parallel_dims.dp_replicate_enabled
+            else ("dp",)
+        )
+        # note that mesh can only be flattened from the finest-grained mesh dimensions
+        dp_mesh = (
+            world_mesh[(*dp_mesh_dim_names, "cp")]._flatten("dp_cp")
+            if parallel_dims.cp_enabled
+            else world_mesh[dp_mesh_dim_names]
+        )
 
         apply_fsdp(
             model,