fix no_grad signature (#23600)

* fix no_grad signature
test=develop

* check func name instead of doc
test=develop

python/paddle/fluid/dygraph/base.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 from ..wrapped_decorator import signature_safe_contextmanager, wrap_decorator
 import contextlib
+import functools
 import sys
 import numpy as np
 from paddle.fluid import core
@@ -195,6 +196,7 @@ def no_grad(func=None):
         return _switch_tracer_mode_guard_(is_train=False)
     else:
 
+        @functools.wraps(func)
         def __impl__(*args, **kwargs):
             with _switch_tracer_mode_guard_(is_train=False):
                 return func(*args, **kwargs)

python/paddle/fluid/tests/unittests/test_imperative_decorator.py

@@ -49,6 +49,7 @@ class TestTracerMode(unittest.TestCase):
             self.tracer._train_mode = self.init_mode
 
             self.assertEqual(self.no_grad_func(1), 1)
+            self.assertEqual(self.no_grad_func.__name__, "no_grad_func")
 
             self.assertEqual(self.tracer._train_mode, self.init_mode)