Update base image in the CI.

[vanbasten23]

Now that #6664 added the missing package toml and we got the new dev image us-central1-docker.pkg.dev/tpu-pytorch-releases/docker/development:3.8_cuda_12.1 and got a new base ci image gcr.io/tpu-pytorch/xla_base:dev-3.8_cuda_12.1_v2, this PR starts using the new base image.

[vanbasten23]

us-central1-docker.pkg.dev/tpu-pytorch-releases/docker/development:3.8_cuda_12.1 image has the missing toml package. But somehow, the image gcr.io/tpu-pytorch/xla_base:dev-3.8_cuda_12.1_v2 doesn't. The image gcr.io/tpu-pytorch/xla_base:dev-3.8_cuda_12.1_v2 was obtained by

(base) xiowei@xiowei-gpu:~/createCIBaseImage$ docker build https://github.com/pytorch/xla.git#master:.circleci/docker
(base) xiowei@xiowei-gpu:~/createCIBaseImage$ docker tag 3558ac0bccf2 gcr.io/tpu-pytorch/xla_base:dev-3.8_cuda_12.1_v2
(base) xiowei@xiowei-gpu:~/createCIBaseImage$ docker image push gcr.io/tpu-pytorch/xla_base:dev-3.8_cuda_12.1_v2