[TensorPipe] Update [Cpu|Cuda]Buffer fwd declarations (#52600)
Summary:
They've changed from class to struct in tensorpipe repo, but have not
been updated in the header, which triggers compiler warning if clang is
used and would have triggered a linker error if the same code is
compiled with MSVC
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52600
Reviewed By: lw
Differential Revision: D26579754
Pulled By: malfet
fbshipit-source-id: 800c02e7ba839bac01adf216de2d8547b7e9128b
