* move operators/math/math_function_* to pten/kernels/func
* namespace from `paddle::operators::math` to `pten::funcs`

* fix extra op for expand, expand_as, tile, unstack

* fix unique unstack dim 0

* Update expand_v2_op.cc

* fix unique_op format

* update doc, test=document_fix

* add attr(dtype)

* refine code

* add unique_v2 op

* remove unique_v2 op

* update doc

Those ops include，scale， sum， sums，unique_with_counts，unique，
wherre， add error message and test case

* test=develop
Add the op of unique_with_counts, the op is calc the unqiue input of data, and output the corresponding indices and count of data.

* test=develop
Check the input and dtype in the op of unique_with_counts

* test=develop
test=document_preview
update the API.spec for `unique_with_counts`, at the same time, optimize the python api in the op of `unique_with_count`

* test=develop
test=document_preview
Fix some python api problem in the op of `unique_with_counts`, and change the error messsage in this op.

* Fix some API problem in the op of `unique_with_counts`
test=develop
test=document_preview

* test=develop
test=document_preview
Fix the api sample of op `unique_with_counts`, and update api.spec