I thought `mask_zero=True` will output 0's when the input value is 0, so the following layers could skip computation or something.

How does `mask_zero` works? 

Example: 

```
data_in = np.array([
  [1, 2, 0, 0]
])
data_in.shape
>>> (1, 4)

# model
x = Input(shape=(4,))
e = Embedding(5, 5, mask_zero=True)(x)

m = Model(inputs=x, outputs=e)
p = m.predict(data_in)
print(p.shape)
print(p)

```

The actual output is: (the numbers are random)

```
(1, 4, 5)
[[[ 0.02499047  0.04617121  0.01586803  0.0338897   0.009652  ]
  [ 0.04782704 -0.04035913 -0.0341589   0.03020919 -0.01157228]
  [ 0.00451764 -0.01433611  0.02606953  0.00328832  0.02650392]
  [ 0.00451764 -0.01433611  0.02606953  0.00328832  0.02650392]]]

```

However, I thought the output will be:

```
[[[ 0.02499047  0.04617121  0.01586803  0.0338897   0.009652  ]
  [ 0.04782704 -0.04035913 -0.0341589   0.03020919 -0.01157228]
  [ 0 0 0 0 0]
  [ 0 0 0 0 0]]]

```