Add ATen export and gradient for torch.max/min (#11275)
* add aten export for max, max.dim
* rewrite grad of max (no dim); add cases for min
* update UT cases
* mod sym shape infer
* resolve comments: shape infer, add comments, etc.
* add test for torch.max of two tensors
* resolve peng's comments: keepdim; test case
* correct python format
* fix recently introduced lint error
