Teams: allow split stride of zero iff size is 1

[davidozog]

This PR allows team split operations to have a stride of zero if and only if the new team size is 1.

[wrrobin]

LGTM. Minor change to update team->stride to 0 when PE_stride is ignored with PE-size = 1 can be done.

[davidozog]

@wrrobin @philipmarshall21 - I reverted the zero-stride checks across collectives (998f7c9) by forcing stride=1 internally, which I think is a little cleaner. Can you please re-review?

(Also this updates the SOS tests-sos submodule to point to the latest commit).

[wrrobin]

LGTM. So, internally, we are not allowing stride to be 0. Perhaps, we should discuss if we need to revisit this in OpenSHMEM.

[davidozog]

LGTM. So, internally, we are not allowing stride to be 0. Perhaps, we should discuss if we need to revisit this in OpenSHMEM.

Yes, agreed. Fortunately I think the spec is arguably well-defined if the following is true: shmem_team_split_strided allows stride to be zero only if size is 1 (but stride can also be any value when size is 1). So it's up to library implementers to handle the special zero-stride case, because it's likely to lead to divide-by-zero exceptions.

I've also drafted a change here that should help clarify this in the spec:
davidozog/openshmem-specification@010e424

I will propose it for v1.6 if there's enough time...

[philipmarshall21]

LGTM!