Enable PJRT plugins by default

[will-cromar]

GOOGLE_CUDA is defined in bazel by OpenXLA. This will be unset by default. Therefore, we'll use dynamic plugins by default, enabling automatic discovery of the CUDA plugin if you're using the normal/tpu build.

Tested:

# python
>>> import os
>>> os.environ['TF_CPP_MIN_LOG_LEVEL'] = '0'
>>> os.environ['TF_CPP_VMODULE'] = 'pjrt_registry=5'
>>> import torch_xla
2024-06-13 17:39:34.217260: I torch_xla/csrc/runtime/pjrt_registry.cc:70] Registering PjRt plugin NEURON
2024-06-13 17:39:34.217363: I torch_xla/csrc/runtime/pjrt_registry.cc:70] Registering PjRt plugin TPU
2024-06-13 17:39:34.217400: I torch_xla/csrc/runtime/pjrt_registry.cc:70] Registering PjRt plugin XPU
>>> 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.
2024-06-13 17:39:39.318121: I torch_xla/csrc/runtime/pjrt_registry.cc:83] Initializing client for PjRt plugin TPU
WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
I0000 00:00:1718300379.343529 1209222 pjrt_api.cc:99] GetPjrtApi was found for tpu at /usr/local/lib/python3.10/site-packages/libtpu/libtpu.so
I0000 00:00:1718300379.343613 1209222 pjrt_api.cc:78] PJRT_Api is set for device type tpu
I0000 00:00:1718300379.343620 1209222 pjrt_api.cc:145] The PJRT plugin has PJRT API version 0.54. The framework PJRT API version is 0.54.
2024-06-13 17:39:42.233641: I external/xla/xla/pjrt/pjrt_c_api_client.cc:127] PjRtCApiClient created.
device(type='xla', index=0)

Enables #7249

Nothing changes with the old CUDA build.

# PJRT_DEVICE=CUDA python
Python 3.10.14 (main, May 14 2024, 08:51:34) [GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> os.environ['TF_CPP_MIN_LOG_LEVEL'] = '0'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined
>>> import os
>>> os.environ['TF_CPP_MIN_LOG_LEVEL'] = '0'
>>> os.environ['TF_CPP_VMODULE'] = 'pjrt_registry=5'
>>> import torch_xla
>>> torch_xla.devices()
2024-06-13 19:55:16.605772: I torch_xla/csrc/runtime/pjrt_registry.cc:147] Initializing PjRt GPU client...

See #6242