Support TPU Pod

[infwinston]

This PR enables TPU Pod usage. To change from single TPU and TPU pod, user only needs to modify accelerators: tpu-v2-8 to accelerators: tpu-v2-32.

sky launch and sky exec will sync filemount/run setup/execute codes on all the TPU pod nodes.

Note: GCP does not support stopping a tpu pod ref.

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TODO:

• [x] Run the MNIST example with tpu-v2-32 and tpu-v3-32

sky launch examples/tpu/tpuvm_mnist.yaml --gpus tpu-v2-32 -c podv2
sky launch examples/tpu/tpuvm_mnist.yaml --gpus tpu-v3-32 -c podv3

• [ ] Write a tpu pod test