Fix test_nss.py import error when ng_model_gym is not installed (pytorch#17184)

Summary:

Fixes test collection failure in CI when `ng_model_gym` is not installed.

The `test_nss.py` file imports `ng_model_gym` at module level, which requires `pydantic`. In CI environments where these dependencies are not installed, pytest fails during test collection with `ModuleNotFoundError: No module named 'pydantic'`.

This fix adds `pytest.importorskip("ng_model_gym")` before the import, which gracefully skips the entire test module when the dependency is unavailable instead of failing test collection.
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Differential Revision: D92206283

Author: Andrew Pullin

backends/arm/test/models/test_nss.py

@@ -7,7 +7,6 @@
 
 import pytest
 import torch
-
 from executorch.backends.arm.test import common
 from executorch.backends.arm.test.tester.test_pipeline import (
     EthosU55PipelineINT,
@@ -16,9 +15,11 @@
     TosaPipelineINT,
     VgfPipeline,
 )
-
 from huggingface_hub import hf_hub_download
 
+# Skip entire module if ng_model_gym is not installed (requires pydantic)
+pytest.importorskip("ng_model_gym")
+
 from ng_model_gym.usecases.nss.model.model_blocks import (  # type: ignore[import-not-found,import-untyped]
     AutoEncoderV1,
 )