[TOPI] Add layer norm operator

[vinx13]

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This PR added a tuple-sum based implementation of layer norm. It performs one-pass reduction to compute mean and variance at the same time. Reducer pattern is also added to allow LowerCrossThreadReduction to handle this case. On CUDA, it will generate two kernels: one for reduction and one for elemwise operations. Because of some limitation of compute_at currently we are not able to fuse them into one kernel.

cc @MasterJH5574 @junrushao @AndrewZhaoLuo