Fix output dtype of elementwise_div

[linjieccc]

PR types

Bug fixes

PR changes

Others

Describe

整数除法结果还是整数：

import paddle

a = paddle.to_tensor([327])
b = paddle.to_tensor([80])
a / b
'''
Tensor(shape=[1], dtype=int64, place=CUDAPlace(0), stop_gradient=True,
       [4])
'''

修复后结果为浮点数：

import paddle

a = paddle.to_tensor([327])
b = paddle.to_tensor([80])
a / b
'''
Tensor(shape=[1], dtype=float32, place=Place(gpu:0), stop_gradient=True,
       [4.08750010])
'''

[wawltor]

加上注释、以及单测

[wawltor]

LGTM

[wawltor]

LGTM