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733389c6 - Refactor isinstance(x, ConstantVariable) to x.is_python_constant() (#169006)

Refactor isinstance(x, ConstantVariable) to x.is_python_constant() (#169006) Summary: X-link: https://github.com/pytorch/pytorch/pull/169006 Approved by: https://github.com/zou3519, https://github.com/anijain2305 Reviewed By: izaitsevfb Differential Revision: D88657870 fbshipit-source-id: dee0b89dc89ef76a76217978130287ffb7f8e43b