MLBEDSW-3509: Updated the debug database to support multiple custom operators.

Previously the debug database lost some operators in the debug database outputs when multiple custom operators were generated by Vela.
Also, the file offsets for command streams were always 0, even for a single custom operator. This patch should rectify these problems.

Signed-off-by: erik.andersson@arm.com <erik.andersson@arm.com>
Change-Id: Ieb072440d4f1806d4833a676683b4f42f431f3df
diff --git a/ethosu/vela/nn_graph.py b/ethosu/vela/nn_graph.py
index 71d4e61..db878bc 100644
--- a/ethosu/vela/nn_graph.py
+++ b/ethosu/vela/nn_graph.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2020 Arm Limited or its affiliates. All rights reserved.
+# Copyright (C) 2020-2021 Arm Limited or its affiliates. All rights reserved.
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -150,6 +150,7 @@
         self.flash_tensor = None
         # Scratch information locally used in the scheduler
         self.scheduling_info = {}
+        self.generated_stream_id = None
 
         self.memory_used = {}
         self.memory_used_per_type = {}