Improve device auto-detection

[will-cromar]

Device auto-detection only works for some functions.

torch_xla.device() works:

# python
>>> import torch_xla
>>> torch_xla.device()
WARNING:root:PJRT is now the default runtime. For more information, see https://github.com/pytorch/xla/blob/master/docs/pjrt.md
WARNING:root:libtpu.so and TPU device found. Setting PJRT_DEVICE=TPU.
device(type='xla', index=0)

torch_xla.real_devices() does not work:

# python
>>> import torch_xla
>>> torch_xla.real_devices()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/workspaces/ptxla/pytorch/xla/torch_xla/torch_xla.py", line 49, in real_devices
    return torch_xla._XLAC._xla_real_devices()
RuntimeError: torch_xla/csrc/runtime/runtime.cc:31 : $PJRT_DEVICE is not set.

Only functions that pass through a function wrapped in requires_pjrt (or ones that call using_pjrt) trigger auto-detection; we did this to accommodate XRT, which is no longer a concern. I believe we can trigger auto detection upon import, or at least more broadly to cover our public API usage.

Tasks:

• Remove using_pjrt and requires_pjrt. These functions are both irrelevant now, and we only (ab)use them for a side-effect
• Remove this warning during auto-detection: WARNING:root:PJRT is now the default runtime. For more information, see https://github.com/pytorch/xla/blob/master/docs/pjrt.md
• Trigger device auto-detection somewhere else that makes sense (e.g. package import). Remember that anything that references env vars will cause a graph break in Dynamo according to @JackCaoG

[zpcore]

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?

[will-cromar]

__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.