Created by: lijianshe02

Add nll_loss op, paddle.nn.loss.NLLLoss(weight=None, reduction='mean', ignore_index=-100).

examples:

        import paddle.fluid as fluid
        import numpy as np

        input_np = np.random.random(size=(10, 10)).astype(np.float32)
        label_np = np.random.randint(0, 10, size=(10, )).astype(np.int64)
        prog = fluid.Program()
        startup_prog = fluid.Program()
        place = fluid.CPUPlace()
        with fluid.program_guard(prog, startup_prog):
            input = fluid.data(name='input', shape=[10, 10], dtype='float32')
            label = fluid.data(name='label', shape=[10], dtype='int64')
            nll_loss = paddle.nn.loss.NLLLoss()
            res = nll_loss(input, label)

            exe = fluid.Executor(place)
            static_result = exe.run(
                prog,
                feed={"input": input_np,
                      "label": label_np},
                fetch_list=[res])
        print(static_result)