Propose a typedefs header for paddle framework

[reyoung]

The original discuess in

• scope design doc #2548 (comment)
• init tensor design doc #2579 (comment)

This commit is just a proposal, let's do such kind of summarization in
this PR.

[reyoung]

I just run

grep std (find . -name '*.h' -o -name '*.cpp') | sed -r 's#.*std\:\:([a-z_0-9]+)[<(\ ].*#\1#g' | grep -v 'std::' | sort | uniq -c | sort -r | head -n 10

in current PaddlePaddle repo. It returns

1116 vector
 601 string
 289 make_shared
 278 unique_ptr
 140 shared_ptr
  82 function
  76 setw
  75 dynamic_pointer_cast
  70 pair
  63 min

Maybe we should typedef:

• map/set because the unordered_map/unordered_set is faster than default map/set. And unordered_ is too long.
• unique_ptr/shared_ptr/make_unique(which in c++ 14 standard, but can be implemented in C++ 11 manually)/make_shared/dynamic_pointer_cast because it is heavily used and there are many smart pointer implementations.
• vector/string/function/pair because they are heavily used.

And there are some classes in boost are very useful.

• boost::variant and boost::any. They are in C++ 17 standard.
• boost::non_copyable. Mark a class is non-copyable. Instead of using a macro, using this base class would not pollute the global namespace.

[gangliao]

you did not distinguish special case, for instance, is_array<T>::value

For that case, you need to using U = typename remove_extent<T>::type;

[gangliao]

auto a_uptr = make_unique<int[]>(10);

[JiayiFeng]

How about UMap or uMap?
I'm afraid most people will assume Map to be the alias of std::map at first sight.

[wangkuiyi]

I think using std::vector saves readers the time to grep and find out what Vector is.

Also, I don't think we will always use std::map or std::unordered_map over another one. Instead, in some curcumstances we need std::map and in some others we need std::unordered_map. It doesn't make much sense to me to stick on either of them and give the stick a name like Map.