caffe2: expose CPUContext RandSeed for backwards compatibility with external RNG (#43239)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43239
This is an incremental step as part of the process to migrate caffe2 random number generator off of std::mt19937 and to instead use at::mt19937+at::CPUGeneratorImpl. The ATen variants are much more performant (10x faster).
This adds a way to get the CPUContext RandSeed for tail use cases that require a std::mt19937 and borrow the CPUContext one.
Test Plan: This isn't used anywhere within the caffe2 codebase. Compile should be sufficient.
Reviewed By: dzhulgakov
Differential Revision: D23203280
fbshipit-source-id: 595c1cb447290604ee3ef61d5b5fc079b61a4e14
