underscore_attrs_are_private and validate_all not compatible with validate_arguments

[xbanke]

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.2
            pydantic compiled: True
                 install path: /home/xxx/.conda/envs/py38/lib/python3.8/site-packages/pydantic
               python version: 3.8.5 (default, Sep  4 2020, 07:30:14)  [GCC 7.3.0]
                     platform: Linux-4.18.0-80.el8.x86_64-x86_64-with-glibc2.10
     optional deps. installed: ['typing-extensions']

underscore_attrs_are_private not work with validate_arguments just like it not work with create_model #3134 .

When I define a function decorated by validate_arguments with validate_all been set True, It will raise ValidationError:

from datetime import datetime
from pydantic import validate_arguments, Field


@validate_arguments(config={'validate_all': True})
def foo(dt: datetime = Field(default_factory=lambda : 42)):
    print(type(dt))

foo()

---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
<ipython-input-4-6c97ede6fd47> in <module>
      7     print(type(dt))
      8 
----> 9 foo()

~/.conda/envs/py38/lib/python3.8/site-packages/pydantic/decorator.cpython-38-x86_64-linux-gnu.so in pydantic.decorator.validate_arguments.validate.wrapper_function()

~/.conda/envs/py38/lib/python3.8/site-packages/pydantic/decorator.cpython-38-x86_64-linux-gnu.so in pydantic.decorator.ValidatedFunction.call()

~/.conda/envs/py38/lib/python3.8/site-packages/pydantic/decorator.cpython-38-x86_64-linux-gnu.so in pydantic.decorator.ValidatedFunction.init_model_instance()

~/.conda/envs/py38/lib/python3.8/site-packages/pydantic/main.cpython-38-x86_64-linux-gnu.so in pydantic.main.BaseModel.__init__()

ValidationError: 3 validation errors for Foo
v__duplicate_kwargs
  object of type 'NoneType' has no len() (type=type_error)
args
  object of type 'NoneType' has no len() (type=type_error)
kwargs
  object of type 'NoneType' has no len() (type=type_error)