Update opset 15 version supported ops doc (#1773)

* update opset 15 doc

Signed-off-by: hwangdeyu <[email protected]>

* add the opset15 unit test

Signed-off-by: hwangdeyu <[email protected]>

Author: hwangdeyu

ci_build/azure_pipelines/templates/unit_test.yml

@@ -1,7 +1,7 @@
 # Run unit test
 
 parameters:
-    onnx_opsets: ['14', '13', '12', '11', '10', '9', '8']
+    onnx_opsets: ['15', '14', '13', '12', '11', '10', '9', '8']
     skip_tflite_tests: 'True'
     skip_tfjs_tests: 'True'
     skip_tf_tests: 'False'