Fix PP+CP handling freqs_cis buffer

When developing test_pp_cp and chatting with @fegin, we realized the
freqs_cis buffers are not being handled correctly in torchtitan for the
pipelining case.

CP needs to modify the freqs_cis buffer to account for sharding on seq
dim, but in the previous titan code this was implemented incorrectly.
`model.freqs_cis` was passed to CP for sharding, but pipelining does not
use `model` at all, it uses the different stage-models contained in
`model_parts` list.  The fix is to tell CP context about each freqs_cis
buffer inside `model_parts` models.

Alternatively we could tie the freqs_cis buffers for each pp stage
together, by explicitly doing so after calling init_weights per
pp-stage.  However this is of limited value so we skip it.

[ghstack-poisoned]

train.py

@@ -154,6 +154,8 @@ def loss_fn(pred, labels):
         pp_schedule, model_parts = models_pipelining_fns[model_name](
             model, pp_mesh, parallel_dims, job_config, device, model_config, loss_fn
         )
+        # when PP is enabled, `model` obj is no longer used after this point, model_parts is used instead
+        del model
 
         # For PP with looped schedules, each item in model_parts is one stage-model-chunk.
         # We need to iterate through model_parts to apply SPMD parallelisms, compilation,
@@ -269,11 +271,12 @@ def loss_fn(pred, labels):
             optimizers.zero_grad()
 
             # apply context parallelism if cp is enabled
+            # ensure CP handles the separate freqs_cis buffer for each pp stage
             optional_context_parallel_ctx = (
                 utils.create_context_parallel_ctx(
                     cp_mesh=world_mesh["cp"],
-                    cp_buffers=[input_ids, labels, model.freqs_cis],
-                    cp_seq_dims=[1, 1, 0],
+                    cp_buffers=[input_ids, labels] + [m.freqs_cis for m in model_parts],
+                    cp_seq_dims=[1, 1] + [0 for _ in model_parts],
                     cp_no_restore_buffers={input_ids, labels},
                     cp_rotate_method=job_config.experimental.context_parallel_rotate_method,
                 )