Clarify branch policy of torch-xpu-ops repo - what's viable/strict branch?

[dvrogozh]

@EikanWang : can you, please, help clarify branch policy of torch-xpu-ops repo?

In particular, I see that PyTorch main branch currently points to the https://github.com/intel/torch-xpu-ops/tree/viable/strict of torch-xpu-ops while I was thought it should point to main unless we stabilize code prior to release in which case it would point to one of release branches.

• https://github.com/pytorch/pytorch/blob/3f9f6048dabeb2adce6dd112371c3caff36e8c39/third_party/xpu.txt#L1

$ git branch -a --contains b3d5d78c72ea
  remotes/origin/viable/strict