[TensorExpr] remove Let and LetStmt in favour of binding in Block (#37606)
Summary:
Implementation of the less popular proposal for eliminating overlap between LetStmt and Let: removing both and storing a mapping between Var and value Expr in the Block.
This complicates some tests but simplifies the IR by restricting where variable binding can occur.
I used the unit tests & python integration tests to verify this is correct but I'm unsure of coverage, particularly around the dependency checker in loopnest - ZolotukhinM your review would be useful there.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37606
Differential Revision: D21467483
Pulled By: nickgg
fbshipit-source-id: b402d3fce4cacf35d75f300f0a7dca32a43b6688
