fix(mgb/index): enable index desc empty

GitOrigin-RevId: 4f0ab7c6e7b214b110586df80b8e56eea42f7e3d

src/opr/impl/internal/indexing_helper.cpp

@@ -91,8 +91,6 @@ FancyIndexingHelper::FancyIndexingHelper(
 
 void FancyIndexingHelper::init(const IndexDesc &index_desc) {
     mgb_assert(input().size() == m_idx_inp_start);
-    mgb_throw_if(index_desc.empty(), GraphError,
-            "empty index desc for subtensor opr");
     mgb_assert(m_index_desc.empty());
 
     m_input2idxonly_axis_indexer.resize(input().size(), nullptr);

src/opr/include/megbrain/opr/internal/indexing_helper_sereg.h

@@ -85,7 +85,7 @@ namespace serialization {
         static cg::OperatorNodeBase* load(
                 OprLoadContext &ctx, const cg::VarNodeArray &inputs,
                 const OperatorNodeConfig &config) {
-            mgb_assert(inputs.size() >= 2);
+            mgb_assert(inputs.size() >= 1);
             auto index_desc = ctx.read_param<IndexDescMaskDump>().
                 to_index_desc(inputs.begin() + 1, inputs.end());
             return Opr::make(inputs[0], index_desc, config).node()->owner_opr();
@@ -106,7 +106,7 @@ namespace serialization {
         static cg::OperatorNodeBase* load(
                 OprLoadContext &ctx, const cg::VarNodeArray &inputs,
                 const OperatorNodeConfig &config) {
-            mgb_assert(inputs.size() >= 3);
+            mgb_assert(inputs.size() >= 2);
             auto index_desc = ctx.read_param<IndexDescMaskDump>().
                 to_index_desc(inputs.begin() + 2, inputs.end());
             return Opr::make(inputs[0], inputs[1],