[Bugfix][TIR][VTA] Update host-side target, even without device func (#14982)

This resolves an issue introduced by the combination of
#14918 and
#14945.  The bug occurred for
targets that do not require device-side codegen, but do require a
`device_type` other than `kDLCPU`.  It wasn't caught by CI, as the
issue only occurred with the combination of both PRs.

1. #14918 updated `SplitHostDevice` to only modify the `"target"`
   attribute when a device-side function has been extracted.

2. For VTA, there is no device-side function, as everything is done
   through host-side API calls.

3. From (1) and (2), the VTA examples kept the target
   `T.target("ext_dev", host="llvm")` after the `SplitHostDevice`
   pass, instead of being updated to `T.target("llvm")`.

4. #14945 restricted CombineContextCall to only apply to host-side
   passes.

5. From (4) and (5), the `CombineContextCall` pass was no longer
   applied to the VTA context calls.

This PR fixes `SplitHostDevice`, updating the target from
`T.target("ext_dev", host="llvm")` to `T.target("llvm")`, even if no
device sections have been extracted from the function.

Author: Lunderberg

src/tir/transforms/split_host_device.cc

@@ -108,12 +108,12 @@ PrimFunc SplitHostDevice(PrimFunc func, IRModule* device_mod, const GlobalVar& g
 
   HostDeviceSplitter splitter(device_mod, name_prefix);
 
-  auto body = splitter(func->body);
-
-  if (!body.same_as(func->body)) {
+  if (auto body = splitter(func->body); !body.same_as(func->body)) {
     func.CopyOnWrite()->body = body;
-    auto target_host = target->GetHost().value_or(Target("llvm"));
-    func = WithAttr(std::move(func), tvm::attr::kTarget, target_host);
+  }
+
+  if (auto target_host = target->GetHost()) {
+    func = WithAttr(std::move(func), tvm::attr::kTarget, target_host.value());
   }
 
   return func;

tests/python/unittest/test_tir_transform_split_host_device.py

@@ -168,5 +168,21 @@ def main_kernel(n: T.int32):
         return mod
 
 
+class TestSplitHostDevice(BaseCompare):
+    """Like TestSplitHostDevice, but no device regions to extract
+
+    Even if there are no device regions, the host-side function should
+    still have its "target" attribute updated.
+    """
+
+    def before():
+        T.func_attr({"target": T.target("ext_dev", host="llvm")})
+        T.evaluate(0)
+
+    def expected():
+        T.func_attr({"target": T.target("llvm")})
+        T.evaluate(0)
+
+
 if __name__ == "__main__":
     tvm.testing.main()