Update onnx coverage script for more accurate result (#15029)
Summary:
The coverage of scalar-input test cases were not accurate. This patch fixed that.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15029
Differential Revision: D13419764
Pulled By: zrphercule
fbshipit-source-id: a14a5cbef432bea8c9126156f5deb1125e1aeb47
