add label key `volcano.sh/task-index`

[shuaiyy]

Is your feature request related to a problem? Please describe.
current, volcano task pod has label:

volcano.sh/job-name
volcano.sh/task-spec=worker

but missed task-index

task-index is important:

• get task-index from pod object label, instead of parsing pod name
• add a service by selector job-name task-spec task-index if someone need a k8s Service for a target task Pod
• mount env from labels as a TASK_INDEX; the env plugin provide env key VC_TASK_INDEX which user cannot custom the name.

Describe the solution you'd like

add a volcano.sh/task-index to pod label

Describe alternatives you've considered

I want to know the reason why task-index not set to pod annotation

Additional context

the kubeflow training-operator project provider those labels in the task pod:

training.kubeflow.org/job-name
training.kubeflow.org/replica-type=chief
training.kubeflow.org/replica-index=0