[Caffe2] Create fewer strings during argument fetching (#64285)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64285
With C++14 heterogeneous ordered container lookup, it is no longer necessary to create a `std::string` in order to look up elements of a `CaffeMap` keyed by std::string. Accordingly, this diff reworks the argument-getting operator functions to avoid that in favor of `c10::string_view`.
ghstack-source-id: 137139818
ghstack-source-id: 137139818
Test Plan: buildsizebot iOS apps -- code size win. less strings is probably marginally good for perf but this only happens at setup time anyway.
Reviewed By: dzhulgakov
Differential Revision: D26826676
fbshipit-source-id: ee653b14dc2c528bae8c90f0fc6a7a419cbca1d6
