[TRANSFORM] Fix virtual device anaotation issue with BYOC subgraphs

Heterogeneous module partitioned by BYOC has functions nodes without any VirtualDevice
definition (having FullyUnconstrained device). Ignoring the device here causes
expr_virtual_devices_ being empty when PopVirtualDevice is called assuming above PushVirtualDevice
is succeeded. PushVirtualDevice and PopVirtualDevice occurs as pairs across function body,
hence it's better to insert the The Virtual Device for Uncontrained and Pop it subsequently.

src/relay/transforms/device_aware_visitors.cc

@@ -94,9 +94,6 @@ void LexicalOnDeviceMixin::ExitFunctionBody() {
 }
 
 void LexicalOnDeviceMixin::PushVirtualDevice(const VirtualDevice& virtual_device) {
-  if (virtual_device->IsFullyUnconstrained()) {
-    return;
-  }
   expr_virtual_devices_.emplace_back(virtual_device);
 }