Fix 8gpu PP failure due to 2D DCP disablement

DCP recently added safeties to avoid using it for 2D/3D since strided
sharding (a feature needed for safe 2D/3D resharding) is not ready yet.

PP uses DCP to load a seed checkpoint.  Disabling the safety mechanism
is enough to make 3D/PP still work (for the case where we train from the
beginning or do not re-shard.

(Resharding refers to saving a checkpoint from one world
size/parallelism config and loading/resuming under a different one).

ghstack-source-id: c069d2186c79517c72f5b3c99485cebdc15df08f
Pull Request resolved: pytorch#460

torchtitan/parallelisms/parallelize_llama.py

@@ -507,6 +507,17 @@ def apply_fsdp(
         model, **fsdp_config, reshard_after_forward=not parallel_dims.pp_enabled
     )
 
+    if parallel_dims.pp_enabled:
+        # TODO
+        # This PR https://github.com/pytorch/pytorch/pull/129519 added a safety check to avoid using 2D/3D DCP since
+        # without strided sharding, DCP can not safely support resharding for 2D/3D.  However, for PP to work, even
+        # without resharding, we load a seed-checkpoint and need to disable the safety mechanism.  This hack should be
+        # removed after strided sharding is landed in DCP.
+        for module in model.modules():
+            assert len(module._load_state_dict_pre_hooks) <= 1
+            module._load_state_dict_pre_hooks.clear()
+            assert len(module._state_dict_pre_hooks) <= 1
+            module._state_dict_pre_hooks.clear()
     logger.info("Applied FSDP to the model")
     return model