feat(training): add --config-path to config validate and unify config composition

[frazane]

Description

Add --config-path/-i to anemoi-training config validate so a config living in any directory can be validated without first cd-ing into it, and route both config validate and config dump through a single Hydra-composition helper. This brings validate in line with train (which honours Hydra's --config-path) and dump (which already exposes --config-path/-i). Closes #1184.

What problem does this change solve?

A new, backward-compatible feature plus an internal cleanup.

validate was the only config-consuming subcommand with no way to say where the config lives: it exposed only --config-name, --overwrite and --mask_env_vars, and composed with an empty primary path (initialize(version_base=None, config_path="")), so a config was reachable only through AnemoiSearchPathPlugin — the current working directory and the packaged defaults. Validating a config anywhere else meant changing directory first.

When --config-path is given, the config is now composed from that directory via Hydra's initialize_config_dir, which installs it as the primary (main) search-path entry — the same precedence @hydra.main gives train's --config-path: --config-path > current working directory > packaged defaults (the model #1181 established). When it is omitted, behaviour is unchanged, and the misleading config_path="" (which did no useful work) is dropped.

anemoi-training config validate --config-path /path/to/configs --config-name debug.yaml

What issue or task does this change relate to?

Closes #1184. Follows #1181, which made the user-specified --config-path take precedence and simplified the search-path hierarchy for train.

Additional notes

validate and dump now share one composition helper — initialize_config_dir(dir) when a --config-path is given, plain initialize() otherwise. This replaces dump's previous approach of copying the directory's files into a temporary directory and chdir-ing into it, a workaround that existed only because Hydra's initialize resolves config_path relative to the calling module and so cannot take an arbitrary user directory. dump's observable behaviour is unchanged — it still composes the named config from the given directory, or from the working directory and packaged defaults when none is given — but it no longer copies files or mutates the process working directory.

Tests: tests/unit/commands/test_config.py covers validate composing a config discovered only via --config-path, and dump composing from a --config-path directory. Docs: a note and example were added to docs/user-guide/configuring.rst.

This change was developed in tandem with AI coding agents.

---

As a contributor to the Anemoi framework, please ensure that your changes include unit tests, updates to any affected dependencies and documentation, and have been tested in a parallel setting (i.e., with multiple GPUs). As a reviewer, you are also responsible for verifying these aspects and requesting changes if they are not adequately addressed. For guidelines about those please refer to https://anemoi.readthedocs.io/en/latest/

By opening this pull request, I affirm that all authors agree to the Contributor License Agreement.

---

📚 Documentation preview 📚: https://anemoi-training--1216.org.readthedocs.build/en/1216/

---

📚 Documentation preview 📚: https://anemoi-graphs--1216.org.readthedocs.build/en/1216/

---

📚 Documentation preview 📚: https://anemoi-models--1216.org.readthedocs.build/en/1216/