When we init a key after another key pushed. This key has no merged_buf_ in file 'comm.h', but the inited_ is true. So it can't pull this new key. 
```
import mxnet as mx
a=mx.nd.array([1,2,3], ctx=mx.gpu(0))
b=mx.nd.array([0,0,0], ctx=mx.gpu(0))
kv=mx.kv.create('device')
kv.init('1', a)
kv.push('1', [a,a,a,a])
kv.pull('1', b)
kv.init('2', a)
kv.pull('2', b)
```