Fix MPI Test runnable errors

[tenzen-y]

What this PR does / why we need it:

The Validate() function is no more acceptable for JobSetTemplate as a dedicated argument. However, when we merge #2555, unfortunately added the no available arguments to our MPI UTs

# github.com/kubeflow/trainer/pkg/runtime/framework/plugins/mpi [github.com/kubeflow/trainer/pkg/runtime/framework/plugins/mpi.test]
Error: pkg/runtime/framework/plugins/mpi/mpi_test.go:821:93: too many arguments in call to p.(framework.CustomValidationPlugin).Validate
	have (nil, *"github.com/kubeflow/trainer/pkg/runtime".Info, *"github.com/kubeflow/trainer/pkg/apis/trainer/v1alpha1".TrainJob, *"github.com/kubeflow/trainer/pkg/apis/trainer/v1alpha1".TrainJob)
	want (*"github.com/kubeflow/trainer/pkg/runtime".Info, *"github.com/kubeflow/trainer/pkg/apis/trainer/v1alpha1".TrainJob, *"github.com/kubeflow/trainer/pkg/apis/trainer/v1alpha1".TrainJob)

https://github.com/kubeflow/trainer/actions/runs/14106238715/job/39513354634#step:4:91

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[tenzen-y]

/assign @kubeflow/wg-training-leads @astefanutti

[astefanutti]

LGTM

[andreyvelich]

Thanks @tenzen-y!
/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andreyvelich, astefanutti

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