test: add TensorFlow graph reset in teardown method for entrypoint tests and bias standard tests (#5049)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Improved test isolation by resetting the TensorFlow default graph
before and after affected tests to ensure a clean graph state between
runs.
* Adjusted test modules to import the TensorFlow environment wrapper
used in tests.
* No changes to test assertions or runtime behavior beyond lifecycle
improvements.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: OutisLi

source/tests/tf/test_nvnmd_entrypoints.py

@@ -515,6 +515,7 @@ def test_model_qnn_v0(self) -> None:
     def tearDown(self) -> None:
         # close
         nvnmd_cfg.enable = False
+        tf.reset_default_graph()
 
 
 class TestNvnmdEntrypointsV1(tf.test.TestCase):
@@ -878,6 +879,7 @@ def test_wrap_qnn_v1(self) -> None:
     def tearDown(self) -> None:
         # close
         nvnmd_cfg.enable = False
+        tf.reset_default_graph()
 
 
 if __name__ == "__main__":

source/tests/tf/test_out_bias_std.py

@@ -7,6 +7,9 @@
 from deepmd.tf.descriptor.se_a import (
     DescrptSeA,
 )
+from deepmd.tf.env import (
+    tf,
+)
 from deepmd.tf.fit.dipole import (
     DipoleFittingSeA,
 )
@@ -21,6 +24,16 @@
 class TestOutBiasStd(unittest.TestCase):
     """Test out_bias and out_std functionality in TensorFlow backend."""
 
+    def setUp(self):
+        """Resets the default graph before each test."""
+        super().setUp()
+        tf.reset_default_graph()
+
+    def tearDown(self):
+        """Resets the default graph after each test."""
+        tf.reset_default_graph()
+        super().tearDown()
+
     def test_init_out_stat_basic(self):
         """Test basic init_out_stat functionality."""
         descriptor = DescrptSeA(