Refine op_types_to_quantize argument handling in matmul_4bits_quant…

…izer.py (#21815)

### Description
<!-- Describe your changes. -->

Refine `op_types_to_quantize` argument handling in
matmul_4bits_quantizer.py

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
The default `op_types_to_quantize "MatMul"` will cause
`tuple(args.op_types_to_quantize)` to become `('M', 'a', 't', 'M', 'u',
'l')`, which is not expected.

onnxruntime/python/tools/quantization/matmul_4bits_quantizer.py

@@ -1062,7 +1062,6 @@ def parse_args():
     )
     parser.add_argument(
         "--op_types_to_quantize",
-        default="MatMul",
         type=str,
         nargs="+",
         choices=["MatMul", "Gather"],
@@ -1089,7 +1088,7 @@ def parse_args():
     input_model_path = args.input_model
     output_model_path = args.output_model
     quant_format = QuantFormat[args.quant_format]
-    op_types_to_quantize = tuple(args.op_types_to_quantize) if args.op_types_to_quantize else None
+    op_types_to_quantize = tuple(args.op_types_to_quantize) if args.op_types_to_quantize else ("MatMul",)
     quant_axes = tuple(args.quant_axes) if args.quant_axes else None
 
     if os.path.exists(output_model_path):