[TensorExpr] Simplify conditional select (#43350)
Summary:
Fold conditional select when both sides are constant.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43350
Test Plan: test_tensorexpr --gtest_filter=TensorExprTest.ConditionalSelectFold*
Reviewed By: pbelevich
Differential Revision: D23256602
Pulled By: asuhan
fbshipit-source-id: ec04b1e4ae64f59fa574047f2d7af55a717a5262
