fix dlwp_healpix constant coupler batch inference

[ivanauyeung]

Modulus Pull Request

Description

Fix the bug where dlwp_healpix constant coupler did not support batch coupled inputs for inference

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The CHANGELOG.md is up to date with these changes.
• An issue is linked to this pull request.

Dependencies

[dallasfoster]

What test covers this change?

[yairchn]

@ivanauyeung
I tested successfully the inference of ensemble forecast with this PR.
However, do we know this works in training? have you tested that?

[ivanauyeung]

The function set_coupled_fields in the coupler.py is only used in the inference stage. So these changes will not affect the training. Also, tests are added to cover this function.