docstr: Test deprecated_arg decorator with just `since` set.
docstr_tok: ["Test","deprecated_arg","decorator","with","just","`","since","`","set","."]
code: def test_arg_warn1(self):
      """Test deprecated_arg decorator with just `since` set."""
  
      @deprecated_arg(
          "b", since=self.prev_version, version_val=self.test_version
      )
      def afoo1(a, b=None):
          pass
  
      afoo1(1)  # ok when no b provided
  
      self.assertWarns(FutureWarning, lambda: afoo1(1, 2))
  
imports: from __future__ import annotations
  import unittest
  import warnings
  from monai.utils import DeprecatedError, deprecated, deprecated_arg, deprecated_arg_default
label_desc: Use image node_id 12 for calling the TestDeprecated obj's underlying member method code with example usage: obj.test_arg_warn1() without return types