Enable dynamism on xla::add (#4868)
* created another SymIntElements ctor
* add some change
* add more logging.
* the result shape is correct.
* the first test passed.
* throw error if one operand is dynamic and one is static with non-one dimension.
* added a third test and it passed.
* remove prints
* fix rebase conflict
* fix up
* fix comments
* relax xla check.
* fix comments
