[train][Docs] Add Backblaze B2 example + alias B2_APPLICATION_KEY env vars onto AWS_*

[goanpeca]

Description

Aliases B2_APPLICATION_KEY_ID / B2_APPLICATION_KEY (the env vars the Backblaze b2 CLI uses) onto AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY so pyarrow's S3FileSystem picks them up when resolving s3:// URIs. Without this, users following Backblaze's own docs see credentials silently ignored.

Implementation: _alias_s3_compatible_credentials_to_aws_env_vars() in python/ray/train/_internal/storage.py, called from get_fs_and_path(). No-op if any AWS-named var is already set; warns on a partial pair.

Also adds a Backblaze B2 example to the S3-compatible storage docs section, linking to a runnable notebook at https://github.com/backblaze-b2-samples/notebooks/tree/main/ray-train-tune-checkpoints.

Related issues

#63104

Additional information

get_fs_and_path() already honors AWS_ENDPOINT_URL_S3 via pyarrow's FileSystem.from_uri (see https://arrow.apache.org/docs/cpp/env_vars.html); the alias just closes the credential-naming gap. 4 new unit tests in python/ray/train/tests/test_storage.py. No new dependencies.

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[Copilot]

Pull request overview

This PR improves Ray Train’s S3-compatible storage experience (notably Backblaze B2) by ensuring credentials provided via Backblaze’s CLI env var names are made visible to pyarrow’s S3 resolver, and by expanding the docs with a B2-focused example.

Changes:

• Add _alias_s3_compatible_credentials_to_aws_env_vars() and call it from get_fs_and_path() to map B2 env vars onto AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY when appropriate.
• Add unit tests covering aliasing behavior, no-op behavior, and warnings.
• Update Train persistent-storage docs with a Backblaze B2 example, endpoint override guidance, and a link to an end-to-end notebook.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
python/ray/train/_internal/storage.py	Adds credential env var aliasing logic and invokes it during filesystem resolution.
python/ray/train/tests/test_storage.py	Adds tests validating the new env var aliasing behavior and logging.
doc/source/train/user-guides/persistent-storage.rst	Updates S3-compatible storage docs to include Backblaze B2 guidance and a runnable example link.

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