Checking for the image in the manifest of TrainingRuntime

### What happened?

Now there is a check for the image in the TrainingRuntime manifest, although the spec itself does not require this. We want to use a single TrainingRuntime manifest to run multiple images that use the same resources using CustomTrainer or CustomTrainerContainer, which contain the image attribute.
https://github.com/kubeflow/sdk/blob/0.4.0/kubeflow/trainer/backends/kubernetes/utils.py#L106

### What did you expect to happen?

The image check will be performed when creating Trainer CR.
https://github.com/kubeflow/sdk/blob/0.4.0/kubeflow/trainer/backends/kubernetes/utils.py#L426

### Environment

Kubernetes version:
```bash
$ kubectl version
Client Version: v1.35.3
Kustomize Version: v5.7.1
Server Version: v1.33.0
Warning: version difference between client (1.35) and server (1.33) exceeds the supported minor version skew of +/-1
```
Kubeflow Trainer version:
```bash
$ kubectl get pods -n kubeflow -l app.kubernetes.io/name=trainer -o jsonpath="{.items[*].spec.containers[*].image}"

```
Kubeflow Python SDK version:
```bash
$ pip show kubeflow
0.4.0
```

### Impacted by this bug?

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Start in kubeflow/trainer/backends/kubernetes/utils.py, comparing the image check near line 106 with the Trainer CR creation path near line 426. Trace when TrainingRuntime and CustomTrainer or CustomTrainerContainer manifests are validated; done means image validation occurs when creating the Trainer CR, not because TrainingRuntime itself requires an image.

Written by the indexing model from the issue text.