Add nightly C++11 ABI builds
Initially, PyTorch/XLA was built with C++11 ABI, which was a more modern
ABI (e.g. small string optimizations). However, that conflicts with the
ABI of upstream PyTorch, so https://github.com/pytorch/xla/pull/5650
disabled the C++11 ABI. It turns out that C++11 ABI improves LazyTensor
tracing performance drastically for some large models. We can't go back
to C++11 ABI due to conflict with upstream, so this PR adds additional
nightly build variants that enable C++11 ABI next to the existing wheels
and dockers.
The docker images will be tagged like "nightly_3.10_tpuvm_cxx11_20241023"
instead of "nightly_3.10_tpuvm_20241023".
The wheels will be named like
"torch_xla-2.6.0.dev20241023.cxx11-cp310-cp310-linux_x86_64.whl" instead
of "torch_xla-2.6.0.dev20241023-cp310-cp310-linux_x86_64.whl".
This PR also adds support for building C++11 ABI variants for stable
branches, but we don't activate it yet.
This PR also removes Python 3.8 nightly builds.
Fixes #8306.
