Select fully async rollout with --fully-async

[guapisolo]

Motivation

Selecting the fully async rollout should be part of the argument surface, not an environment variable. The previous approach (MILES_FULLY_ASYNC=1) rewrote sys.argv before parse_args() to inject --rollout-function-path, which meant a hand-rolled argparse prefix matcher to detect conflicts, validation split across two entrypoints, and configuration that never appears in --help or in the logged config table.

Retargeted onto #1717 (fully-async rewritten as FullyAsyncRolloutFn).

What this PR does

Adds --fully-async, resolved in miles_validate_args where the rest of the cross-argument checks already live:

- python3 train.py ...
+ MILES_EXPERIMENTAL_ROLLOUT_REFACTOR=1 python3 train_async.py ...
+   --fully-async

_validate_fully_async(args) asserts:

• class-based rollout API required — MILES_EXPERIMENTAL_ROLLOUT_REFACTOR=1, otherwise the class entry point cannot be loaded at all;
• no simultaneous --rollout-function-path — detected by comparing against default_rollout_function_path() (the default is now a named helper instead of an inline conditional), so no argv scanning;
• no --colocate — the async driver rejects colocation, and failing at argument time beats failing after the placement group is built.

It then sets rollout_function_path, and points eval_function_path at the standard inference rollout unless the user set it explicitly — FullyAsyncRolloutFn raises on eval, and eval otherwise inherits the rollout path. That makes the flag a complete switch: the launch scripts and the e2e harness no longer spell out two function paths by hand.

train.py asserts the flag is off, so choosing the wrong driver fails loudly instead of silently running synchronously.

Testing

Launch scripts, docs, and the 30B fully-async e2e case (#1717) converted to the flag, so the e2e run exercises it end to end. pre-commit run --all-files passes.

🤖 Generated with Claude Code

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.