Use type-erased union for Buffer. (#322)
Summary:
Pull Request resolved: https://github.com/pytorch/tensorpipe/pull/322
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54145
In order to merge the channel hierarchies, we need a generic `Buffer` type, that can wrap either a `CpuBuffer` or a `CudaBuffer`.
The constraints are that, since this type is used by the channels, it cannot explicitly refer to `CudaBuffer`. We propose here a type-erasure based solution, with small-buffer optimization to avoid heap-allocating the wrapped concrete buffer.
ghstack-source-id: 124131499
Test Plan: CI
Reviewed By: lw
Differential Revision: D27001339
fbshipit-source-id: 26d7dc19d69d7e3336df6fd4ff6ec118dc17c5b6
