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81294ff0 - Refactor isinstance(x, TensorVariable) to x.is_tensor() (#169441)

Refactor isinstance(x, TensorVariable) to x.is_tensor() (#169441) Summary: X-link: https://github.com/pytorch/pytorch/pull/169441 Approved by: https://github.com/zou3519 ghstack dependencies: #169006, #169341 Reviewed By: izaitsevfb Differential Revision: D88657932 fbshipit-source-id: 774e1c95b8460adb49f93fecf29cbac44ef63dfb