[TensorExpr] Add aten::sum lowering to the kernel (#43585)
Summary:
Handles all dimensions and selected dimensions, per PyTorch semantics.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43585
Test Plan: test_tensorexpr
Reviewed By: bertmaher
Differential Revision: D23362382
Pulled By: asuhan
fbshipit-source-id: e8d8f1197a026be0b46603b0807d996a0de5d58c
