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fix: Fix crash when using cp in dtensor path #1663

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Merged
terrykong merged 1 commit into from
Dec 19, 2025
Merged

fix: Fix crash when using cp in dtensor path #1663

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16 changes: 15 additions & 1 deletion nemo_rl/models/policy/workers/dtensor_policy_worker_v2.py
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Original file line number Diff line number Diff line change
Expand Up @@ -277,10 +277,24 @@ def __init__(
# All ranks initialize model on meta device, so FSDP can shard it.
# The actual weights will be broadcast from rank 0.

cp_size = self.cfg["dtensor_cfg"]["context_parallel_size"]
with init_empty_weights():
# NeMoAutoModelForCausalLM uses flash_attention_2 by default
# so we need to set it to None if sequence packing is disabled
# https://github.com/NVIDIA-NeMo/Automodel/blob/7e748be260651349307862426c0c168cebdeeec3/nemo_automodel/components/_transformers/auto_model.py#L180
if cp_size > 1:
# Match Automodel's `get_train_context` in `cp_utils.py` where only
# flash and efficient backends are supported
# Ref: https://github.com/NVIDIA-NeMo/Automodel/blob/81788d6f4848f5f066c4a6a2bece4689a6a83687/nemo_automodel/components/distributed/cp_utils.py#L57
from torch.nn.attention import SDPBackend

sdpa_method = [
SDPBackend.FLASH_ATTENTION,
SDPBackend.EFFICIENT_ATTENTION,
]
else:
sdpa_method = None

self.model = model_class.from_config(
model_config,
attn_implementation="flash_attention_2"
Expand All @@ -289,6 +303,7 @@ def __init__(
use_liger_kernel=False,
trust_remote_code=True,
torch_dtype=str(model_config.torch_dtype),
sdpa_method=sdpa_method,
)
if self.lora_enabled:
apply_lora_to_linear_modules(self.model, self.peft_config)
Expand All @@ -297,7 +312,6 @@ def __init__(
self.model.config.pad_token_id = tokenizer.pad_token_id

tp_size = self.cfg["dtensor_cfg"]["tensor_parallel_size"]
cp_size = self.cfg["dtensor_cfg"]["context_parallel_size"]
if cp_size > 1 and self.enable_seq_packing:
raise ValueError(
"Context parallel is not supported for sequence packing. Refer to https://github.com/NVIDIA/NeMo-RL/blob/main/docs/model-quirks.md#context-parallel-with-fsdp2 for more details."
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