[ROCm][GHA] split 4 GPU hosts across two runners
Examine the runner name. If it ends with "-2" to indicate the second runner on the host, the docker run arguments will select the last two GPUs. Otherwise, select the first two GPUs on the host.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76849
Approved by: https://github.com/janeyx99
