Fix pyre error 3

Co-authored-by: cccclai <[email protected]>

Author: YifanShenSZ

extension/llm/export/partitioner_lib.py

@@ -55,7 +55,7 @@ def get_mps_partitioner(use_kv_cache: bool = False):
     return MPSPartitioner(compile_specs)
 
 
-def get_coreml_partitioner(use_kv_cache: bool = False, pt2e_quantize: str = None):
+def get_coreml_partitioner(use_kv_cache: bool = False, pt2e_quantize: Optional[str] = None):
     assert (
         use_kv_cache is True
     ), "CoreML backend currently only supports static shape and use_kv_cache=True is the only way to support it at the moment"