MLBEDSW-3144: Add weights compression ratio

  - Also removed the original bit_per_element

Change-Id: I51bfbd28e14f316aae2d542bb610a3ed57b8b53b
Signed-off-by: Diqing Zhong <diqing.zhong@arm.com>
diff --git a/ethosu/vela/nn_graph.py b/ethosu/vela/nn_graph.py
index d2c848a..71d4e61 100644
--- a/ethosu/vela/nn_graph.py
+++ b/ethosu/vela/nn_graph.py
@@ -512,9 +512,9 @@
         self.subgraphs = []
         self.metadata = []
         self.memory_used = {}
-        self.bits_per_element = {}
-        self.total_size = {}
-        self.total_elements = {}
+        self.weights_compression_ratio = 0
+        self.total_original_weights = 0
+        self.total_compressed_weights = 0
         self.weight_cache = None  # See CompressedWeightCache
 
     def get_root_subgraph(self):