[Core] Support custom executor qualname

[22quinn]

Purpose

Some RL frameworks may define its own custom executor. This PR adds support for passing custom executor qual name via vllm serve and dynamically import the actual class during runtime.
Since v0 is on its way to be deprecated, we added v1 support only.

Test Plan

Unit test: pytest -v tests/v1/executor/test_executor.py
Serving: vllm serve Qwen/Qwen3-0.6B --enforce-eager --distributed-executor-backend="tests.v1.executor.test_executor.CustomMultiprocExecutor"

Test Result

Passed

(Optional) Documentation Update

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• The test plan, such as providing test command.
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[gemini-code-assist]

Code Review

This pull request adds support for specifying a custom executor via its fully qualified name, which is a nice enhancement for framework integrations. The changes are mostly in vllm/v1/executor/abstract.py to handle the dynamic import, and in vllm/config/parallel.py to adjust the configuration validation. A new test file is also added to cover the new functionality. My review has identified a couple of high-severity issues. First, in vllm/v1/executor/abstract.py, there's a potential for an unhandled TypeError if the provided qualname resolves to a non-class object. Second, the validation change in the shared vllm/config/parallel.py file weakens validation for the v0 engine, which could lead to a confusing user experience. I've provided suggestions to address these points.

[gemini-code-assist]

This change weakens the validation for the v0 engine. The v0 engine does not support custom executor paths (qualnames), but this new validation logic allows any string to pass. This can lead to confusing failures later in the initialization process for v0 users. Since this configuration file is shared between v0 and v1, this change is a regression for v0. Could we preserve the stricter validation for v0?

[gemini-code-assist]

The call to issubclass will raise a TypeError if executor_class is not a class (e.g., if the user provides a qualname for a function or module). This would result in an unhandled exception. It's better to validate that executor_class is a type before checking if it's a subclass of ExecutorBase to provide a more user-friendly error message.

Suggested change

	if not issubclass(executor_class, ExecutorBase):
	raise TypeError(
	"distributed_executor_backend must be a subclass of "
	f"ExecutorBase. Got {executor_class}.")
	if not isinstance(executor_class, type) or not issubclass(
	executor_class, ExecutorBase):
	raise TypeError(
	"distributed_executor_backend must be a class and a subclass of "
	f"ExecutorBase. Got {executor_class}.")

[22quinn]

false positive. resolve_obj_by_qualname will return the class or raise error within the function.