fix bug in 'device' type kvstore (#12350)

* fix bug in 'device' type kvstore

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)
```

* add kv test pull

src/kvstore/comm.h

@@ -459,6 +459,7 @@ class CommDevice : public Comm {
   void Init(int key, const NDArrayStorageType stype, const TShape& shape,
             int dtype = mshadow::kFloat32) override {
     sorted_key_attrs_.emplace_back(key, shape, dtype);
+    inited_ = false;
   }
 
   void InitBuffersAndComm(const std::vector<NDArray>& src) {
@@ -701,8 +702,10 @@ class CommDevice : public Comm {
       }
       // Delayed allocation - as the dense merged buffer might not be used at all if push()
       // only sees sparse arrays
-      bool delay_alloc = true;
-      buf.merged = NDArray(shape, ctx, delay_alloc, type);
+      if (buf.merged.is_none()) {
+        bool delay_alloc = true;
+        buf.merged = NDArray(shape, ctx, delay_alloc, type);
+      }
       ctx_info[ctx.dev_id].second += shape.Size();
     }
     inited_ = true;

tests/python/unittest/test_kvstore.py

@@ -106,6 +106,23 @@ def check_init(kv, key):
     check_init(mx.kv.create(), 3)
     check_init(mx.kv.create(), 'a')
 
+@with_seed()
+def test_pull():
+    """test pull"""
+    def check_pull(kv):
+        a = mx.nd.ones(shape)
+        b = mx.nd.zeros(shape)
+        kv.init('1', mx.nd.zeros(shape))
+        kv.push('1', [a,a,a,a])
+        kv.pull('1', b)
+        check_diff_to_scalar(b, 4)
+        kv.init('2', mx.nd.zeros(shape))
+        kv.pull('2', b)
+        check_diff_to_scalar(b, 0)
+
+    check_pull(mx.kv.create('device'))
+    check_pull(mx.kv.create())
+
 @with_seed()
 def test_list_kv_pair():
     """list key-value pair push & pull"""