Change the default argument for Calibration Tasks for GPTQ

Summary:
GPTQ is the sole user of the `Calibration_Tasks` param. Previously the default value was [], which is not something `Int8DynActInt4WeightGPTQQuantizer` handles gracefully.

This made the field effectively always required for GPTQ. By setting it to None, we allow quantize to resolve to ["wikitext"] and save a little bit of typing

Differential Revision: D55458866

Author: Jack-Khuu

examples/models/llama2/export_llama_lib.py

@@ -341,8 +341,7 @@ def build_args_parser() -> argparse.ArgumentParser:
     parser.add_argument(
         "--calibration_tasks",
         nargs="+",
-        type=str,
-        default=[],
+        default=None,
         help="Tasks for GPTQ calibration",
     )
     parser.add_argument(