mx.nd.contrib.boolean_mask doesn't support GradientAddition

[wkcn]

Description

Hi, there.
I found mx.nd.contrib.boolean_mask gives a wrong gradient when gradient addition on it.
mx.nd.contrib.boolean_mask doesn't support Gradient Addition.

Environment info (Required)

----------Python Info---------- 
Version      : 3.7.3        
Compiler     : GCC 8.2.1 20181127
Build        : ('default', 'Mar 26 2019 21:43:19')
Arch         : ('64bit', 'ELF')   
------------Pip Info-----------                       
Version      : 19.0.3     
Directory    : /usr/lib/python3.7/site-packages/pip
----------MXNet Info-----------                                                                                               
Version      : 1.5.0                                                                                                    
Directory    : /home/wkcn/proj/incubator-mxnet/python/mxnet                                                                   Hashtag not found. Not installed from pre-built package.                                                                      ----------System Info----------                                                                                       
Platform     : Linux-5.1.6-arch1-1-ARCH-x86_64-with-arch                                                                      
system       : Linux                                           
node         : MiraiT                                          
release      : 5.1.6-arch1-1-ARCH                                                                                             
version      : #1 SMP PREEMPT Fri May 31 15:17:53 UTC 2019

Package used (Python/R/Scala/Julia):
Python

Build info (Required if built from source)

Compiler (gcc/clang/mingw/visual studio): gcc

MXNet commit hash:
b0be6c5

Build config:
make -j 5 USE_OPENCV=1 USE_BLAS=openblas USE_MKLDNN=0

Minimum reproducible example

import mxnet as mx

a = mx.nd.array([1,2,3,4,5])
a.attach_grad()

bi = mx.nd.array([1,1,0,0,0])

ci = mx.nd.array([1,0,0,1,0])

with mx.autograd.record():
    b = mx.nd.contrib.boolean_mask(a, bi)
    c = mx.nd.contrib.boolean_mask(a, ci)
    su = b.sum() + c.sum()

su.backward()
print(a.grad)
'''
[2.0000000e+00 1.0000000e+00 0.0000000e+00 1.0000000e+00 5.0171812e+04]
<NDArray 5 @cpu(0)>
'''

Steps to reproduce

1. Run the reproduce example
2. The last gradient is wrong and random.

What have you tried to solve it?

1. I read the code, and I found mx.nd.contrib.boolean_mask doesn't support GradAdd.

[mxnet-label-bot]

Hey, this is the MXNet Label Bot.
Thank you for submitting the issue! I will try and suggest some labels so that the appropriate MXNet community members can help resolve it.
Here are my recommended labels: Feature

[wkcn]

The issue has been fixed. Close it.