Masked Summation of Tensor
<SCODE>   99    99
  489  1171
  281  1317
     ⋮      
    0   435
    0  2741
    0   517
[torch.LongTensor of size 2406x2]```

POST EMBEDDING:
```context_output.size() -- (2406L, 2L, 1L)``` -- (word_sequence, batch_size, attention_score)

```Variable containing:
( 0  ,.,.) = 
1.00000e-02 *
  2.0804
  1.6674
  2.9782
   ⋮    
  4.8565
  4.8565
  4.8565
...
( 1  ,.,.) = 
1.00000e-02 *
  0.2246
  1.4224
  4.1816
   ⋮    
  4.4363
  3.0162
  3.3986
[torch.FloatTensor of size 2x2406x1]```


I need to sum the values in the POST EMBEDDING tensor if they have the same ```word_id``` from the PRE EMBEDDING tensor.

Example:

PRE EMBEDDING
```Variable containing:
   4    1
   4    2
   2    2
[torch.LongTensor of size 3x2]```

POST EMBEDDING:

```Variable containing:
( 0  ,.,.) =
  0.35
  0.35
  0.65
...
( 1  ,.,.) = 
  0.25
  0.65
  0.65
[torch.FloatTensor of size 2x3x1]```

DESIRED RESULT

```Variable containing:
( 0  ,.,.) =
   4.0    0.7
   2.0    0.65
   1.0    0.0
...
( 1  ,.,.) =
   4.0    0.0
   2.0    1.3
   1.0    0.25
[torch.LongTensor of size 2x3x2]```<ECODE>
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