* Support ignore_index for c_softmax_with_cross_entropy_op.

* Polish code. Remove useless comments and add Testcase.

* Polish code for TestCase.

* Polish code.

* Polish code style.

* Polish code.

* Change loss calculation formula and ignore_index dtype.

* Polish TestCase.

* Fix bug of c_softmax_with_cross_entropy_op_xpu_op. Attribute 'ignore_index'
dtype is int64_t.