Added support L2Normalization op

Added some error checking

python/mxnet/contrib/onnx/mx2onnx/_op_translations.py

@@ -794,6 +794,31 @@ def convert_lrn(node, **kwargs):
     return [lrn_node]
 
 
+@mx_op.register("L2Normalization")
+def convert_l2normalization(node, **kwargs):
+    """Map MXNet's L2Normalization operator attributes to onnx's LpNormalization operator
+    and return the created node.
+    """
+    helper, _, _ = import_onnx_modules()
+    name = node["name"]
+    input_id = kwargs["index_lookup"][node["inputs"][0][0]]
+    input_name = kwargs["proc_nodes"][input_id].name
+    attrs = node["attrs"]
+    mode = attrs.get("mode", "instance")
+
+    if mode != "channel":
+        raise AttributeError("ONNX currently supports channel mode only")
+
+    l2norm_node = helper.make_node(
+        "LpNormalization",
+        [input_name],
+        [name],
+        axis=1,  # channel only
+        name=name
+    )
+    return [l2norm_node]
+
+
 @mx_op.register("Dropout")
 def convert_dropout(node, **kwargs):
     """Map MXNet's Dropout operator attributes to onnx's Dropout operator
@@ -1625,7 +1650,10 @@ def convert_slice_axis(node, **kwargs):
     inputs = node["inputs"]
     axes = int(node["attrs"]["axis"])
     starts = int(node["attrs"]["begin"])
-    ends = int(node["attrs"]["end"])
+    if node["attrs"]["end"] == 'None':
+        raise ValueError("Slice: ONNX doesnt't support 'None' in 'end' attribute")
+    else:
+        ends = int(node["attrs"]["end"])
 
     input_node_id = kwargs["index_lookup"][inputs[0][0]]
     input_node = proc_nodes[input_node_id].name