backward compatible initialization (#666)

## Summary
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this proposed code change. --->
#662 initialized the argument in a way that is not compatible with
python3.9 so changing it to a backward compatible initialization.
This unblocks TRL PR huggingface/trl#3260
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## Details
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## Testing Done
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Author: shivam15s

src/liger_kernel/chunked_loss/grpo_loss.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 import torch
 
 from liger_kernel.chunked_loss.fused_linear_ppo import LigerFusedLinearPPOBase
@@ -204,7 +206,7 @@ def __init__(
         epsilon_low: float = 0.2,
         epsilon_high: float = 0.2,
         loss_type: str = "bnpo",
-        max_completion_length: int | None = None,
+        max_completion_length: Optional[int] = None,
         temperature: float = 1.0,
     ):
         """