[flytekit] Update calling task in task behavior

[wild-endeavor]

User Code

If you have something like this

from flytekit import task, workflow

@task
def outer():
    inner()
    print("outer")

@task
def inner():
    print("inner")

@workflow
def wf():
    outer()

Current Behavior

It works in recent versions of flytekit.

Goal: What should the final outcome look like, ideally?

This pattern should not work without an active decision from the user. We want to actively discourage accidentally doing this for a variety of reasons. One is that the behavior is different from when calling a task within a wf - all the procedures and logic behind a normal task run applies. When you call a task within a task, the idea is that it's executing as if flyte doesn't exist - you're just invoking the underlying function.

Proposal from @cosmicBboy

• when calling a task within a task, raise a deprecation warning saying “this will run as a local function within the parent task, saying this will raise an error in flytekit version X.Y.Z, use <task>. python_function() explicitly to call as a local function”.
• when we release version X.Y.Z raise an error, add migration guide to the release notes / blogs for that release.

We should probably give this at least a few months, maybe half a year, before making it an error.

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes