Use type-erased union for Buffer. (#54251)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54251
Pull Request resolved: https://github.com/pytorch/tensorpipe/pull/324
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.
This is a new version of D27001339 (https://github.com/pytorch/pytorch/commit/c618dc13d2aa23625cb0d7ada694137532a4fa33) which broke PyTorch OSS build.
Test Plan: CI
Reviewed By: lw, mrshenli
Differential Revision: D27156053
fbshipit-source-id: 4244302af33a3be91dcd06093c0d6045d081d3cc
