Improve device auto-detection about xla HOT 2 OPEN

With using_pjrt user doesn't need to call the function _maybe_select_default_device to init the device by themselves. To get rid of using_pjrt decorator, we either need to:

1. force user to call function like _maybe_select_default_device at the beginning of their script; Or
2. place the _maybe_select_default_device within some other functions that must be called at the beginning of their script.

Both ways look annoying. Can we do something in torch_xla/__init__.py file to automatically find the device at the first place?

from xla.

__init__.py sounds fine. That's what I meant by "trigger auto detection upon import".

Broadly, there are two options that I see:

1. Decorate any function that could potentially init the runtime something equivalent to requires_pjrt. This is arguably cleaner/more precise, but I'm concerned we'll miss some functions and not fully root out this $PJRT_DEVICE is not set error.
2. _maybe_select_default_device in __init__.py so it runs automatically when the user imports torch_xla.

I'm open to other solutions too. The core constraint is that runtime init happens in C++, while most of the logic we need to detect devices is in Python.

from xla.