ReduceScatter::eval_cpu: Assert inputs.size = 1

[ronaldmannak]

ReduceScatter::eval_cpu is asserting inputs.size() == 0 while reading the first element, thus aborting valid calls when assertions are enabled.
This PR asserts inputs.size() == 1.

• ☑️ I understand it is strictly prohibited to use AI to write PR description
• AI usage disclosure: Opus 5.5 found this the issue while working on JACCL

[ronaldmannak]

This is ready for review, but it appears there's a limit how many PRs a person can open

[zcbenz]

The limitation is set because we are unable to spend too much time reviewing contributor PRs at the moment, please do not open new PRs before we review your current open ones.

[ronaldmannak]

@zcbenz Got it! I split changes into small PRs thinking it'd make reviewing easier for you :) I have a few more lined up, so I'll wait. BTW, does the one-PR-at-a-time rule also apply to mlx-swift?

[zcbenz]

mlx-swift receives much less PRs so it does not have the restriction, but I think it is generally good practice not to open too much PRs at the same time.