[core] Add a warning when returning an object w/ `num_returns=0`

[edoakes]

What happened + What you expected to happen

If you set num_returns=0, it is likely a mistake if you return a non-None object. We should print a warning message in this case to avoid confusion.

Here is likely where the warning should go (check outputs):

ray/python/ray/_raylet.pyx

Line 4381 in 24ad12d

if num_returns == 0:

Versions / Dependencies

N/A

Reproduction script

>>> import ray
>>> @ray.remote(num_returns=0)
... def f():
...     print('hi')
...     return 123
...
>>> f.remote()
(f pid=44332) hi

Issue Severity

Low: It annoys or frustrates me.

[edoakes]

Note we can't make this case raise an exception as it would be a breaking change.

[edoakes]

Let's print a warning like:

Task '(name)' has num_returns=0 but returned a non-None value '(repr of object)'. The return value will be ignored.

[justinrmiller]

I'd like to work on this.