test: add test test_missing_tensorrt_package.

Author: GdoongMathew

tests/tests_pytorch/models/test_torch_tensorrt.py

@@ -1,15 +1,27 @@
 import os
+import re
 from io import BytesIO
 from pathlib import Path
 
 import pytest
 import torch
 
 import tests_pytorch.helpers.pipelines as pipes
+from lightning.pytorch.core.module import _TORCH_TRT_AVAILABLE
 from lightning.pytorch.demos.boring_classes import BoringModel
 from tests_pytorch.helpers.runif import RunIf
 
 
+@pytest.mark.skipif(_TORCH_TRT_AVAILABLE, reason="Run this test only if tensorrt is not available.")
+def test_missing_tensorrt_package():
+    model = BoringModel()
+    with pytest.raises(
+        ModuleNotFoundError,
+        match=re.escape(f"`{type(model).__name__}.to_tensorrt` requires `torch_tensorrt` to be installed. "),
+    ):
+        model.to_tensorrt("model.trt")
+
+
 @RunIf(tensorrt=True, min_cuda_gpus=1, min_torch="2.2.0")
 def test_tensorrt_saves_with_input_sample(tmp_path):
     model = BoringModel()