* Compatible int32 and int64 for attr in op slice/strided_slice. test=develop

* Polish code in nn.py  test=develop

* Fix bug: set the same dtype for the inputs of elementwise_add. test=develop

* Convert int32 to int64 in slice op to avoid data overflow. test=develop

* Convert int32 to int64 in strided_slice_op to avoid data overflow. test=develop

* add support for value tensor support of fill_constant Op

* support to modify dict and list in while_loop test=develop

* polish code test=develop

* refine code, test=develop

* reuse contain_var, test=develop