[air] fix pyarrow lazy import

[matthewdeng]

Why are these changes needed?

#35938 introduced a pyarrow typehint, though pyarrow is being lazily imported. This leads to the following error:

>>> from ray import tune
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/matt/miniconda3/envs/test-tune/lib/python3.9/site-packages/ray/tune/__init__.py", line 2, in <module>
    from ray.tune.tune import run_experiments, run
  File "/Users/matt/miniconda3/envs/test-tune/lib/python3.9/site-packages/ray/tune/tune.py", line 27, in <module>
    from ray.air import CheckpointConfig
  File "/Users/matt/miniconda3/envs/test-tune/lib/python3.9/site-packages/ray/air/__init__.py", line 1, in <module>
    from ray.air.checkpoint import Checkpoint
  File "/Users/matt/miniconda3/envs/test-tune/lib/python3.9/site-packages/ray/air/checkpoint.py", line 22, in <module>
    from ray.air._internal.remote_storage import (
  File "/Users/matt/miniconda3/envs/test-tune/lib/python3.9/site-packages/ray/air/_internal/remote_storage.py", line 142, in <module>
    _cached_fs: Dict[tuple, Tuple[float, pyarrow.fs.FileSystem]] = {}
AttributeError: 'NoneType' object has no attribute 'fs'

Note that functionality that does rely on pyarrow will call the following utility function and raise a descriptive error if it is not installed:

ray/python/ray/air/_internal/remote_storage.py

Lines 102 to 109 in b385a0b

	def _assert_pyarrow_installed():
	if pyarrow is None:
	raise RuntimeError(
	"Uploading, downloading, and deleting from cloud storage "
	"requires pyarrow to be installed. Install with: "
	"`pip install pyarrow`. Subsequent calls to cloud operations "
	"will be ignored."
	)

Related issue number

Closes #37656

Checks

Verified locally that running from ray import tune succeeds.

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[rickyyx]

are we cherry picking this?