[Test] Add tvm.testing.requires_libtorch (apache#12737)

Create a specific test dependency to map to USE_LIBTORCH, which
is disabled by deafult, and is independent from torch being
installed on the underlying machine, so it causes problems in
machines that have torch installed but TVM is build with
USE_LIBTORCH OFF.

Mark tests.python.contrib.test_libtorch_ops.test_backend with
this new decorator.

python/tvm/testing/utils.py

@@ -945,6 +945,9 @@ def _any_gpu_exists():
 # Mark a test as requiring Arm(R) Ethos(TM)-N to run
 requires_ethosn = Feature("ethosn", "Arm(R) Ethos(TM)-N", cmake_flag="USE_ETHOSN")
 
+# Mark a test as requiring libtorch to run
+requires_libtorch = Feature("libtorch", "LibTorch", cmake_flag="USE_LIBTORCH")
+
 # Mark a test as requiring Hexagon to run
 requires_hexagon = Feature(
     "hexagon",

tests/python/contrib/test_libtorch_ops.py

@@ -19,6 +19,7 @@
 
 import tvm.relay
 from tvm.relay.op.contrib import torchop
+from tvm.testing import requires_libtorch
 
 import_torch_error = None
 
@@ -30,6 +31,7 @@
 
 
 @pytest.mark.skipif(torch is None, reason=f"PyTorch is not available: {import_torch_error}")
+@requires_libtorch
 def test_backend():
     @torch.jit.script
     def script_fn(x, y):