Fix Merge Bugs

* There is a merge conflict when merge PR #2914
* Develop and PR #2914 both add `DDim::size` method, but did not
  triger git merge conflict while merge.

paddle/framework/ddim.cc

@@ -280,8 +280,6 @@ std::ostream& operator<<(std::ostream& os, const DDim& ddim) {
   return os;
 }
 
-ssize_t DDim::size() const { return arity(*this); }
-
 DDim::DDim(std::initializer_list<int> init_list) {
   *this = make_ddim(init_list);
 }

paddle/framework/ddim.h

@@ -52,8 +52,6 @@ struct DDim {
 
   DDimVar getVar() { return var; }
 
-  ssize_t size() const;
-
   bool operator==(DDim d) const;
 
   bool operator!=(DDim d) const;