SAC API follow ups to restore old behavior (#401)

see #397

torchtitan/parallelisms/parallelize_llama.py

@@ -45,17 +45,26 @@
 # currently selective per op and per layer checkpointing are supported
 def checkpoint_wrapper(module, config):
     if config.mode == "selective" and config.selective_ac_option == "op":
-        from torch.utils.checkpoint import create_selective_checkpoint_contexts
+        from torch.utils.checkpoint import (
+            CheckpointPolicy,
+            create_selective_checkpoint_contexts,
+        )
 
         def _get_custom_policy(meta):
-            def _custom_policy(mode, func, *args, **kwargs):
+            def _custom_policy(ctx, func, *args, **kwargs):
+                mode = "recompute" if ctx.is_recompute else "forward"
                 mm_count_key = f"{mode}_mm_count"
                 if func == torch.ops.aten.mm.default:
                     meta[mm_count_key] += 1
                 # Saves output of all compute ops, except every second mm
-                return func in no_recompute_list and not (
+                to_save = func in no_recompute_list and not (
                     func == torch.ops.aten.mm.default and meta[mm_count_key] % 2 == 0
                 )
+                return (
+                    CheckpointPolicy.MUST_SAVE
+                    if to_save
+                    else CheckpointPolicy.PREFER_RECOMPUTE
+                )
 
             return _custom_policy