Add workarounds for the temporary absence of exec2.h.

Signed-off-by: Michał Zientkiewicz <mzient@gmail.com>

dali/pipeline/executor/executor2/stream_assignment.h

@@ -26,11 +26,24 @@
 #include <vector>
 #include "dali/pipeline/graph/graph_util.h"
 #include "dali/pipeline/executor/executor2/exec_graph.h"
-#include "dali/pipeline/executor/executor2/exec2.h"
+// TODO(michalz): This is here for review process only. Remove when exec2.h is available
+// #include "dali/pipeline/executor/executor2/exec2.h"
+#include "dali/pipeline/graph/op_graph2.h"
 
 namespace dali {
 namespace exec2 {
 
+// TODO(michalz): This is here for review process only. Remove when exec2.h is available
+enum class StreamPolicy : int {
+  Single,       //< There's just one stream that's used by all operators
+  PerBackend,   //< Operators are scheduled on a stream specific to their backend (mixed or GPU)
+  PerOperator   //< Independent operators are executed on separate streams.
+
+  // TODO(michalz): Check if this is legal with existing operator implementations - likely not
+  // PerIteration, //< Streams are cycled on a per-iteration basis
+};
+
+
 template <StreamPolicy policy>
 class StreamAssignment;