Fix the document for norm op

[emailweixu]

Here is the doc

[emailweixu]

And the name of this op is not very good because it is too general. "cross_channel_norm" might be better

[qingqing01]

Actually, I think a general norm operator is better and this general norm operator can support to do norm in any dimension. A long time ago, @guoshengCS had implemented a general norm operator,
#4496 . This PR used abs_op, reduce_sum_op and pow_op to combine a norm operator in C++ code. But when the net_op was removed, this norm_op was deleted.

And I think a general norm operator can support cross_channel_norm_op, when @sweetsky0901 planed to implement to cross_channel_norm_op, I discussed with her, we should implement a general norm operator.

[lcy-seso]

We need such a general purpose norm operator in the NMT task. A related issue is #7350 . I will discuss with @qingqing01 later to fix this problem.

[lcy-seso]

Because we need a general purpose norm operator. Currently, I am working on this.