Add std::istream variant to Load and LoadOrDie
Add std::istream variant to Load and LoadOrDie

Added for SentencePieceProcessor:
virtual bool Load(std::istream &istream);
virtual void LoadOrDie(std::istream &istream);
google/sentencepiece,google/sentencepiece,tsuchm/sentencepiece,tsuchm/sentencepiece,google/sentencepiece,tsuchm/sentencepiece,tsuchm/sentencepiece,google/sentencepiece