Add llama sdpa to generation script (#4674)

Summary:
After #4024, sdpa is no longer in portable_lib by default, import separately.


Differential Revision: D61163753

Author: lucylq

examples/models/llama2/runner/generation.py

@@ -17,6 +17,11 @@
 from executorch.examples.models.llama2.tokenizer.tiktoken import Tokenizer
 from executorch.extension.pybindings.portable_lib import _load_for_executorch
 
+from executorch.extension.pybindings import portable_lib  # noqa # usort: skip
+
+# Note: import this after portable_lib
+from executorch.extension.llm.custom_ops import sdpa_with_kv_cache  # noqa # usort: skip
+
 
 class CompletionPrediction(TypedDict, total=False):
     generation: str