How to set the reference tensor of crop operator?

[reyoung]

The documentation comment of the crop operator does not contain the format of the reference tensor.

Paddle/paddle/operators/crop_op.cc

Lines 77 to 110 in 9592468

	Crop Operator.
	Crop input into output, as specified by offsets and shape.
	There are two ways to set shape:
	1. reference input: crop input X into the same shape as reference input.
	The dimension of reference input should
	be the same as the dimension of input X.
	2. shape list: crop input X into the shape described by a list<int>.
	The size of shape list should be the same as
	the dimension size of input X.
	The input should be a k-D tensor(k > 0 and k < 7). As an example:
	Given:
	X = [[0, 1, 2, 0, 0]
	[0, 3, 4, 0, 0]
	[0, 0, 0, 0, 0]],
	and
	offsets = [0, 1],
	and
	shape = [2, 2],
	we get:
	Out = [[1, 2],
	[3, 4]].
	)DOC");

We need add the documentation.