OpenXLA CUDA fallback: Tracking Issue

[ysiraichi]

PR #7318 introduced OpenXLA CUDA fallback. However, that's an opt-in feature that relies on the environment variable: XLA_FALLBACK_CUDA. Even though that works fine, as this comment suggests, it doesn't provide a good UX. Therefore, we should improve it further by adding the following changes:

• [x] Make OpenXLA CUDA fallback the default fallback, making the environment negating the environment variable to something like DISABLE_XLA_FALLBACK_CUDA #7630
• [ ] Deprecate the environment variable, by making PyTorch/XLA smart enough to pick the appropriate fallback

cc @miladm @JackCaoG @vanbasten23

[ManfeiBai]

Hi, @vanbasten23, is that ok to assign this ticket to you?

[ysiraichi]

I will take on this one. @vanbasten23 let me know if you are already working on any of them.