Corresponding data type for BYTE (pytorch#15627)

Summary:
TensorProto.DataType in caffe2/proto/caffe2.proto has BYTE = 3 defined, while there is no corresponding TypeMeta defined in caffe2/core/types.cc: DataTypeToTypeMeta. This issue failed the C++ tutorial of MNIST + LMDB.
Pull Request resolved: pytorch#15627

Differential Revision: D13709602

Pulled By: ezyang

fbshipit-source-id: d4826d0f9b3975e6a8478d4bad1abbbedcaea197

Author: fulltopic

caffe2/core/types.cc

@@ -37,6 +37,7 @@ const TypeMeta& DataTypeToTypeMeta(const TensorProto::DataType& dt) {
   static std::map<TensorProto::DataType, TypeMeta> type_meta_map{
       {TensorProto_DataType_FLOAT, TypeMeta::Make<float>()},
       {TensorProto_DataType_INT32, TypeMeta::Make<int>()},
+      {TensorProto_DataType_BYTE, TypeMeta::Make<uint8_t>()},
       {TensorProto_DataType_STRING, TypeMeta::Make<std::string>()},
       {TensorProto_DataType_BOOL, TypeMeta::Make<bool>()},
       {TensorProto_DataType_UINT8, TypeMeta::Make<uint8_t>()},