[Fix] Fix serialize NDArray when shape is 0 (#884)

* fix

* fix bugs

include/dgl/runtime/ndarray.h

@@ -440,8 +440,12 @@ inline bool NDArray::Load(dmlc::Stream* strm) {
       << "Invalid DLTensor file format";
   CHECK(data_byte_size == num_elems * elem_bytes)
       << "Invalid DLTensor file format";
+  if (data_byte_size != 0)  {
+    // strm->Read will return the total number of elements successfully read.
+    // Therefore if data_byte_size is zero, the CHECK below would fail.
     CHECK(strm->Read(ret->data, data_byte_size))
         << "Invalid DLTensor file format";
+  }
   if (!DMLC_IO_NO_ENDIAN_SWAP) {
     dmlc::ByteSwap(ret->data, elem_bytes, num_elems);
   }

python/dgl/data/graph_serialize.py

@@ -135,9 +135,9 @@ def load_graphs(filename, idx_list=None):
     >>> glist, label_dict = load_graphs("./data.bin", [0]) # glist will be [g1]
 
     """
-    assert isinstance(idx_list, list)
     if idx_list is None:
         idx_list = []
+    assert isinstance(idx_list, list)
     metadata = _CAPI_DGLLoadGraphs(filename, idx_list, False)
     label_dict = {}
     for k, v in metadata.labels.items():

src/graph/graph_serialize.cc

@@ -191,6 +191,7 @@ StorageMetaData LoadDGLGraphs(const std::string &filename,
                               std::vector<dgl_id_t> idx_list,
                               bool onlyMeta) {
   SeekStream *fs = SeekStream::CreateForRead(filename.c_str(), true);
+  CHECK(fs) << "Filename is invalid";
   StorageMetaData metadata = StorageMetaData::Create();
   // Read DGL MetaData
   uint64_t magicNum, graphType, version;