Add multi-host gpu tests to PyTorch/XLA

[vanbasten23]

Description

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Tests

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[vanbasten23]

hi @will-cromar , currently when I run the test it fails with error The Job "pt-nightly-resnet50-mp-func-v100-x4-699rm" is invalid: spec.template.spec.subdomain: Invalid value: "headless-svc-$(JOB_NAME)": a lowercase RFC 1123 label must consist of lower case alphanumeric characters or '-', and must start and end with an alphanumeric character (e.g. 'my-name', or '123-abc', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?').
The subdomain name (headless-svc-$(JOB_NAME)) is the kubernetes service name. Do you know the correct way to make it depends on the job name?