Custom (MCORE) FSDP interface

nemo/collections/llm/recipes/CONFIGURATION-HIERARCHY.md

@@ -57,6 +57,8 @@
   bucket_size: Optional[int] = None # Maximum number of parameters in each bucket
   average_in_collective: bool = False # If true, compute average in collective directly, as opposed to dividing by the dp_size first and then computing sum in the collective
   fp8_param_gather: bool = False # If true, keep the compute param in fp8 (do not use any other intermediate dtype) and perform the param all-gather in fp8
+  use_custom_fsdp: bool = False # If true, use MCore's custom FSDP implementation. recipe.model.config.gradient_accumulation_fusion must be False when using this
+  data_parallel_sharding_strategy: str = "no_shard" # Data parallel sharding strategy, choices=['no_shard', 'optim', 'optim_grads', 'optim_grads_params']
   ```  
   </blockquote>
   </details>

nemo/lightning/megatron_parallel.py

@@ -689,6 +689,12 @@ def init_ddp(self):
             )  # We need to do this explicitly since this is a attr pytorch uses
             model_chunk.__class__.__getattr__ = getattr_proxy  # type: ignore
 
+            # Ensure that if using custom FSDP, gradient_accumulation_fusion is disabled on the model config.
+            if self.ddp_config.use_custom_fsdp:
+                assert (
+                    module.config.gradient_accumulation_fusion == False
+                ), "gradient_accumulation_fusion must be False when using custom FSDP"
+
         # param_sync_func is set in nemo.lightning.pytorch.optim.megatron
         no_sync_func, grad_sync_func = extract_ddp_funcs(self.ddp_config, self)
         for module in self: